I carried a small backpack with us on our trip last summer, and it was no problem at all on the rides. I would wrap the straps around one of my legs, and I never lost sight of it.
You will want to think about using something to cover it, though, on any rides where it may get wet. Splash Mountain was really the only one that I had to figure up something to cover it - I just used the park map for that, but on future trips, I brought plastic to place over it, and then, I can just put it in the backpack when I am done.
I was also carrying a WDW water/fan bottle with up, and also wrapped that around my legs. Though I never lost it, I did break it on Rockin Roller Coaster, so next time I know to stick that in the bag as well!