Art on a an 3"x5" index card, like this: Index-Card-a-Day 2024 (lots of pics [but not all 61])
Crafty detective sounds like a really fun idea!
I would also love to see more swaps for flat things; it keeps costs down, especially for us mailing to different countries. But I’m also open to bigger swaps 
@jellybean was the Christmas stocking to make a stocking and then put something into it?
I had to look up “fukubukuro” but I like the idea of a surprise bag.
What does “common ground” entail?
I also like the idea of paper dolls for a flat paper swap. The “colour in an envelope” sounds really fun as well!
Here are some more ideas for flat envelope swaps:
- Handmade stationary (OK to fold to put in a standard-size envelope)
- Handmade envelopes (would need to go inside a larger envelope)
- Handmade greeting cards that are blank for sending to others
- Poems (handwritten or typed)
- Stories (handwritten or typed)
- “Dress-up” dolls or other “playtime” paper-based things
- Stickers! You can make some to send and include commercial ones as extras if wished. They could be made by hand, printed and cut out, or made with a cricut)
- Pockets
- Maps
- Scavenger hunts. What about sending a nice handwritten/illustrated scavenger hunt for 10 different things for your partner to find where they live? The exchange could end there, or you could send back the completed hunt to each other.
- Draw something for your partner to colour
- Photograph prints!
- “Surprise single-stamp envelope” - make whatever will fit in an envelope to reach the maxium amount of weight allowed for a single stamp. I did this with @gozer for the “Invite Your Partner Swap” and it was a lot of fun.