Nature's Calling Event: Share Your Gameplay Thread (Ends Sept 2nd)

They are available in the catalogue once you unlock them on one Sim, so it's possible to re-complete later. Here's my plan for my personal save file:

Treat them (and other collectibles) like RL collectible merch. Distribute them throughout various Retail/Small Business lots in your save, like Museum Gift Shops or Camping Supplies Stores, wherever fits for the type of collectible, but don't put them all at the same lot. Have your Sims visit these places throughout their lifetime if there's a collectible they need for their collection. If it's a Small Business, spend the day there. If it's a Retail lot, do something in the open world nearby and browse the shop for a souvenir afterwards.

Examples: 

• Space Rocks could be on "display" at Science Museums, Space Ports (with Rockets), Jewellery Stores or Classes, Alien Theme Park or Spellcaster School.
• Magazines could be set as displays in Libraries, Book Shops, Cafés, Newspaper Office, Fame Agency, Travel Agents etc.
• Wreaths could be available from Florists, Garden Centres, Funeral Homes and Seasonal Décor shops.

Just have it make sense where you put them, and spread them out, making sure they're displayed a Sales Surface/Set for Sale so that visiting Sims can buy them. If it's just for you, maybe move things around occasionally. If you are sharing the Save File or builds with the Limited Time Collectibles, it's like a built in scavenger hunt.