A Holga is probably one of the cheapest and easiest cams to re-purpose.
Just take the lens and square lens holder part off, slap on a piece of aluminum from a soda, beer, etc. can put a tiny hole in it and you're pretty much good to go.
Black tape for a shutter. I've attached a photo of one I hacked last year.
Half the fun of pinhole day is making a camera for it. You've got lots of time to build and experiment before showtime arrives.
Many of us at FW have built cameras. Let us know if we can help, and be sure to show us what you come up with.

Edit!
If you send me your address I"ll send you a Holga body that already has the lens off that you can modify for pinhole if you want it.