I want to do a National Park roadtrip, focusing on Zion, Bryce and the Grand Canyon. I want to hike. I want it to be pretty. I don't want to be cold. If there are areas I can do a river float (not rapids, more of a lovely drifting down a river), that would be fantastic.
What time of year should I go?
I would second Greg's answer that if you go in late August/September you can hike comfortably at the beginning of the day and it only starts getting hot in the afternoon. Being cold would not be a concern at this time of year.
Really anytime late spring through through early fall is fine...it's however busier then and I'd strongly recommend that you stay in the onsite lodging within the Parks (i.e.. Zion Lodge). This prevents needless time wasted getting into or out of the park, plus the lodges provide valuable parking passes that otherwise fill up. If you'd like to see the treks before going on them, they can be seen using an iPad App called TrekFit, which has amazing treks of Zion and Bryce Canyon.
My vote is to go in the late Summer months, like September. The weather is perfect that month. I suggest looking into The Narrows hike. It's a long hike but you're in the river and you need to get a pass to hike there. They are pretty easy to get, just go here http://www.nps.gov/zion/planyourvisit/narrowspermits.htm