A smart phone has a lot of things going for it as a bike computer but it's not for everyone. For starters it's not water proof. If you want to see your speed and other things while you ride, you need a way to mount it on your handlebars or stem. GPS drains phone batteries quickly, so this won't work for something like doing the STP in a day. But you might want to experiment with this because if it works for you it'll be the cheapest option to get a lot of features.

Google My Tracks is free, it will give you data like distance, moving time, and average (but not current) speed. And it will show you a map of where you've been, you can display it in Google Earth. Lots of other phone apps will do this sort of thing, Strava is popular, and you have Google Maps if you want bike directions.

Dedicated bike computers are nice because the battery lasts forever and doesn't affect your phone. Some of them have great features. Mine tells me how much each ride I do affects my cardiovascular fitness and how long I need to rest to fully recover.

Note: the calories on any bike computer are a guess. If you don't wear a chest strap to monitor your heart rate, it's a very wild guess and not very trustworthy.