Doing the millionaire scenario

1. Fishing - Fishing at the pier in Magnolia Promenade is a very lucrative spot. Sims will reliably fish up Dragonfruit, Voodoo dolls and Cowplant berries here, all very valuable! Hint: Don't sell cowberries right away, they are worth even more, over 400 simoleans if you plant them first and then sell them from household inventory! Start with this aspiration first so they get the Collector bonus trait, or any of the Nature aspirations will give the Collector trait. Collectors find more rare and valuable objects. Another great spot is on the Island in Brindleton Bay. Sometimes they will fish up gold bars worth over 6K! The spot I found most reliable for those is down the pathway past the museum and past the bench with the woo hoo bush pets can find feathers in. There is usually some pidgeons hanging out there too. Not the first spot with the boat, the next one with a pier your Sims can't walk on. It's kind of hidden but if you find it, you're golden! (pun intended) ;)
2. Handiness - Have them get this skill maxed out as soon as possible! Once they do, they can go to Granite Falls and get the Camping Mascot schematic to make those on the woodworking table! While there, buy a Glowfruit seed too. Those are the best fertilizer of anything available.
3. Gardening - Don't bother with the less valuable plants, go for things like Bird of Paradise, Dragonfruit, the UFO plant, Orchids, Roses, Lilies, and the Death flower. You'll want some Snapdragons too to graft with Lilies for orchids. Fortunately, a lot of these can be acquired at the Romance Festival in San Myshuno in the right season. I believe it's either Summer or Fall.
4. Collecting - Don't bother opening the mystery boxes found in dirt piles, just sell them. They are worth more that way. However, if you find a treasure map, do use it to get rare MySim Trophies from these boxes. Those are valuable. Find a pond in a park and try to catch some frogs. Sell them all until you get a valuable one and then keep whatever one they find next. But keep trying to catch them until you find the Whirlyflower frog. That one is worth 160 simoleans and a pair can be bred every 3-4 hours. When you find a crystal of low value, keep it and send it in to get an element for the focused mood. Focused is the ideal mood for fishing, gardening and handiness. When they are focused they will skill up faster and when woodworking, the objects they make will be worth more.
5. Painting - Do the aspiration, it's the only way your Sim can sell to a Gallery for more simoleans. Alternately, sell your paintings on Plopsy, especially Masterpieces. I've had some of those go for almost 20K a piece doing that! If you're trying to do this as fast as you can though, don't sell on Plopsy. There is no guarantee they will even sell. You could always take them off Plopsy and sell them if that happens though. In my experience the style of painting that sells for the most is Paint by Reference and any of the Mood paintings. Especially Confident. Always have your Sim inspired while painting by using some of the tips I've listed in tip #6. Their paintings will be worth more simoleans.
6. Moods - As mentioned above focused is one of the best moods you'll need. In addition to elements, Sims can get focused by studying a fossil, ponder at a chess table and hanging a mathematical drawing near the woodworking table. The element can be placed nearby for it and in the garden. Elements can also be placed on Magnolia Pier while fishing or anywhere else they might go fishing. I guess it's best to get more than one element from sending in crytals. Inspired is the best mood for painting. Taking a thoughtful shower, cloud/star gazing and postcards are the best ways to get inspired.
7. Satisfaction Points/Reward Traits - You'll want an easel since painting is a big money maker. As soon as they have enough points, buy the Creative Visionary. Your Sims' paintings will be more valuable with that. Get the Marketing Trait as well for Woodworking so those objects are more valuable as well. Other reward traits to get that will help include; Waterproof so they can garden/fish/collect in the rain, and Steel Bladder. Those are the most important. Save the rest of your points to buy a Money Tree.
8. Money Trees - After you've bought one of these, don't harvest it the first time. Put the ready to be harvested tree in their household inventory. When you do that, they'll get the seeds to plant more trees! This is the only way to get more seeds without the need to buy them from the Rewards store.
9. Lot traits - The best lot trait for painting is Home Studio and Natural Light. For gardening the best are Homey and Great Soil. Homey is also good for the handiness skill. Penny Pixels will give your Sim random simoleans, but I wouldn't bother with that one since you only get three slots for lot traits. My Sim chose Home Studio, Great Soil and Homey for her lot traits.
10. Aspirations - The only aspirations my Sim pursued was Fabulously Wealthy for the Shrewd trait and Painter Extraordinaire so she could sell her paintings to a Gallery. Speaking of Fabulously Wealthy and handiness, the reason my Sim worked toward being able to make the Camp Mascot is because it can be sold from your Sims personal inventory and counts for this aspiration. A lot of Simmers are under the misunderstanding the bathtub is the most valuable. It is, somewhat, but it doesn't count for this aspiration since tubs are put in household inventory. Even placing them on the lot and selling them that way won't work, at least it didn't for me. Camp Mascots are ultimately more valuable since they are close to the same value as tubs, but only take 45 minutes to make, whereas the tubs take 1.5 hours. You want the Shrewd trait because Sims will get an investment amount of simoleans every week based on 5% of their household funds. For instance, my Sims that have over 5 million get an investment check of over 200K every week! It builds quickly.
11. Surge Emotion - This wasn't available when I did this challenge with my previous Sim, but if I were to do it again or go for the million, I would take advantage of it! Once they can use this, take inspired for 12 hours when painting and focused for 12 hours for gardening, fishing and woodworking.
12. Sim Traits - For my Sim I chose; Loves Outdoors for obvious reasons, Loner, so her social need would deteriorate slower and Genius for the random focused moodlet. Her first aspiration was Angling Ace to get the Collector bonus trait.
13. Likes/Dislikes - This wasn't available back then either, but obviously you'll want to choose whatever activities they will be doing to make simoleans as their Likes. That way while doing them, it will get their fun need up without having to resort to using earbuds for fun or using a bike to get their fun up quickly, etc. I wouldn't choose any Dislikes.

Final notes - Since I did this challenge with a single Sim, I turned autonomy off so she wouldn't try to quit whatever she was doing to run off and do something else. I got her the best bed I could as soon as possible to lower the time needed to increase her energy need. I would also definitely upgrade the bed too if all they can afford is a lower energy bed. Upgrading beds was not possible yet when I did this challenge. Always make sure your Sim's needs are as far in the green as possible when crafting objects to sell. The higher their mood, the more valuable the painting, Camp Mascot, etc. will be in the end. My Sim made a couple friends just so she could call them to keep up her social. If it's late, they can still text them for this too. There is also an option to check social media on the phone, this will give a pretty good boost to social as well.

I would not bother buying a lot of expensive objects for their home since they will deteriorate in value as soon as they are placed on the lot, defeating your purpose. My Sim's actual home by the time she had earned the 500K was only worth a little over 21K. They can splurge and go on a spending spree after completing the challenge. Perhaps build a mansion, find a love interest, start a family etc. If you leave aging on for this, they can always buy Potions of Youth to stay young while doing the challenge.