Supplement sheet, or (expensive) have stickers on the cards themselves, maybe the front?
The stickers is the lesser of the two ideas, though; the prompt that connects two paths is something I think would give all cards more "oomph." Maybe. I'm just kicking around something that give people a reason to connect cards to each other?
Alternatively, as we discussed on Twitter, an "overworld map" where you can slot your cards. This could literally be nothing more than an 8.5x11 sheet with a little design and spots that say things like "play with my kids" or "fun to read" or "great art!" or whatever.