Island in the Sky was our favorite district within Canyonlands. Although it was definitely more crowded, you can drive between all of the attractions, and the hikes are all short and easy with just as rewarding payoffs. If you only have the time or energy for one, definitely do Grand View Overlook (at sunset for an extra bonus), because the view of the white rim is stunning.
The Grand View Overlook parking lot is easy to reach by car, and there were plenty of other hikers at this very popular overlook. It's a very easy but scenic trail - Clark and I actually hiked it all in flip flops. If you only have time for one short hike at Island in the Sky, I would recommend this one!