I have a small backpack that I tried last trip, and really liked it! Didn't carry it every day, but it was so nice to have the option.
Be sure to check to see that it's comfortable. Once you buy it, take it home and (before you take the tags off), put the things in that you think you'll be carrying around at WDW (water bottle, jackets, ponchos, snacks, etc.). Then put it on again and wear it around for a while. The first one I bought was very uncomfortable (straps dug into my shoulders when I put things in it), but I had great luck with the second one. I hardly notice I have it on!
As other posters have said, just carry it on rides with you. I even had it on Rock n' Roller Coaster -- Just put it on the floor and put my foot through one of the straps. It stayed in place even when we went upside-down!