The big things like a table and power cord will be there for you but it is the small things you'll need to remember.
A robot cart, extension cord, and some sort of tool organization (toolbox, shelf, etc) are all really, really important.
From a safety perspective (ex-Safety Captain taking over here
), plan on how to safely and securely organize the things you are bringing. Most importantly, plan for how you will charge your batteries. They need to be in a secure, well ventilated area (i.e. not the floor, not covered by other things).
A broom, trash bags, container for safety glasses, boxes for organizing your materials, and a container of some sort to store water so you can stay hydrated will all greatly help you.
The pit spaces are extremely tight and we are all trying to cram a lot of stuff inside. Also, bring a couple of folding chairs. Regionals can be quite taxing on pit crew members!
From an imagery perspective, bring a poster or banner that showcases your team name and colors. You can use PVC pipe to create a standalone piece to hang your flags/posters/banners on. Just remember that regionals have a 10' height limit for pits.