You should really pick up Tom's "Day Hikes From the River" (shameless plug, I know!). There are really an infinite amount of hikes. Pretty much every stop has a side canyon to hike up. Are you considering any Technical Canyoneering?
A few I have not seen mentioned as well:
1. Carbon to Lava 'loop' hike. Best done with a boat shuttle similar to Tapeats/Deer Creek.
2. If you camp at Cardenas, the hike up to Hilltop Ruin is a good one.
3. Vishnu - Go over the Schist Fin into the drainage and go up as far as you are comfortable.
4. Lots of hike variation in Nankoweap. Up Nanko Creek, over the BUtte Fault and down Kwagunt is a great hike. Long day though.... Can be done with a boat shuttle.
5. Fern Glen
7. Tuckup is nice. It is a LONG way to the Conglomerate Arch though!
8. Stone Creek and the upper waterfalls
10. Everything Tom says about Bass (Shinumo) area. You could spend weeks here....
12. Lots of cool hikes could be done up Escalante, 75-mile or Papago Canyons.
If you are considering some technical stuff, there is a whole other list for that...
Have a GReat Trip!