Now I realize that the sort of jokes and pranks you can get at Zonko's or the sort that you make yourself are much more extravagant and incredible then Muggle ones, but I'd like to think that some of these might inspire the two of you with your business.
I picked these up at a Muggle store, so they're not as elaborate as yours, but enjoy them nonetheless.
Happy birthday!
- Lily Potter (& James, though he'd hex me for sending you such rudimentary pranks)