We were out on rented kayaks this weekend and there were a LOT of people out there doing the standup paddling. I was watching them for awhile and you could really see how it would be a good core workout. I wanted to try it but we had limited time. I will say that when the wind picked up pretty heavily in the afternoon, all those stand up people were sitting on their boards and paddling back to the rental place to get out of the wind!

The place we got our kayaks from were renting boards/paddles and PFD's for like $30 for two hours. Sounds like a good way to get an idea if you'll like it before investing in the gear.