Agree with getting Jungle Adventures if you don't have it. Complete the temple adventure and you can find a treasure worth 8000-10000 simoleons. You may also find one that valuable by fishing on the island. And that's on top of all the other treasures you can find. Grab some avocados to plant back home too, they sell for about 8-10 a piece, possibly more if evolved. Also, if one of them masters the archeology skill, they'll get artifacts in the mail to authenticate, and get paid well when they are sent back.
Also have them complete the collector aspiration, then they can sell rocks, elements, & aliens to the Simsonian museum for more money. And have one of them build a rocket ship & complete all of the upgrades. It's slow & expensive, because they need to reach level 10 to complete it & all of the upgrades cost a fair amount of money (so do it while the money makers are still around to finance it), but then they can travel to Sixam which is very rich with treasure, including harvestables that they can plant back home & evolve to a higher value.
Finally, have one of them write books, the higher the skill, the higher the royalties. I forget which genre brings in the most money, I think it's the screenplay. Anyway, write a book a day & watch that royalty check grow! For creative sims, give them the Creative Visionary & Marketable rewards traits, and their works will increase in value. I believe the programming skill can also bring in some money, but it's more time consuming.
