It depends on your needs, really.
1. Power conditioner - It's basially a fancy power strip, but I like the convenience of it.
2. Tuner - If you like a rack-mounted tuner. I found that I actually prefer to have my tuner on my pedalboard.
3. Compressor - Again, it's a personal thing. If I was still using a rack-mount rig, my comp would be a dbx 160A in the rack. As of now, I'm plenty happy with my Demeter Compulator on my pedalboard.
4. Any 1U effects unit (Aguilar AGRO, perhaps?).
5. Blank or vented 1U panel - If you're just looking to fill the space, and you don't NEED anything. If you want to spend more money, and add some lights and knobs, a Funk Logic panel ([Invalid or Expired Link Removed]) might be the call.
5. Sandwich - A mid-gig snack is a handy feature.