Bob, this seems like a fun project. I've had a similar inclination, but it's never gotten off the ground. I'll be watching your progress.
Two things:
http://www.pdatoolbox.com/ is a tool for graphical creation of Palm programs, especially simple ones. It may be worth looking at, but perhaps you're more interested in really getting your hands dirty in the raw code.
Also, as far as programs go, my goal was to write a mileage calculator that could handle multiple cars. I wanted to eventually expand it into a general maintenance tracker. This never got started, but perhaps it might be a little more useful than a tip calculator, with a similar level of complexity?