I am heading to Zion myself in a few weeks. This will be my 3rd trip there. Depending on how many days you want to spend hiking/camping. I would recommend the Trans-Zion trek. The entire route can be completed in 5 days, but there are exits each day should you want to cut it short.
There will need to be permits, but they can be easily obtained if you are willing to stand in line at 4am and be the first. I've done this both previous trips. They hold back several walk in only sites that they wont release until 24hours before the day of the hike.
Really any part of that trail is awesome. My personal favorite was the West Rim and the South west connector trail in Wildcat Canyon.
Check out
http://www.citrusmilo.com/zionguide/that guide helped me tremendously the first time i went.
If you are interested in more info, I can provide you with just about any info you need, from shuttle info to what permits are required and how to get them.