The drive to Kolob reservoir is worth doing. Nice scenery change with higher elevation. We've stayed at Zion for a week several times. Quite a few easy to moderate hikes. The narrows is great but in early May you could have cold or high water. Also with bikes if you want super easy, put bikes on front rack of shuttle (holds 3) ride it up to top, and coast down the 9 miles to the entrance. You can ride the bikes on the Pahrus trail.
That said, if you get antsy to move easily I'd second the option of splitting your camping time with Bryce. I've done the day trip but also recommend camping at Bryce.