I started on a similar idea in 2006, but wasn't good enough to execute it at the time.
My original idea was that you do it ahead of time. It would have general ideas of how long it takes to complete an activity and would essentially provide you a list of maps relative to your hotel. Then, you'd have that information beforehand on your smartphone so that you could avoid roaming charges (which is an issue for international travel.)
The revenue model was funneling to travel sites and possibly travel agents who could provide more customized itineraries.
But now, with OpenStreetMap, you could have an app on the phone. On top of it, it could use the GPS to figure out where you were and put together a slide show for your friends. You could rate when you were there and provide more feedback later if you wanted -- just a simple thumbs up/thumbs down.
I've resigned myself to never getting around to it -- If I'm to found a company, it's going to be in the b2b arena instead.