With reference to User:Nickb (data capture on PDA), ideas:
- press "MWAY", and everything from then on is tagged as motorway
- until you arrive somewhere, get the bike out, press "CYCLE" and tracks from then on is tagged as cycleway
- press "P" to mark a car park without having to do any extra work
- press "CROSS", "RAIL", and a stub of railway gets created perpendicular to your route
How complex do you want to make it? When someone presses "Mark", "P" (parking), then could have buttons for "FREE", "P&D(isplay)", "LAYBY", etc. or "DEFAULT"
Looking at PDA programming languages, displaying maps themselves might be non-trivial...