$20 pair of ALFA antennae. Best and biggest bang for the bucks. (See sticky thread on Antennas).
Custom feet. The stock ones are okay but I say the taller and wider the better to handle variable ground conditions.
Protective clear lens cap for gopro.
Experiment with stock gps module and shield at multiple locations before buying custom module. I personally think the stock Rev B with 3dr V2 shield works great. No need to buy upgrade gps module unless you are having clear problem ie not locking enough satellites to fly in 1 to 4 minutes.
Extra lighting. One of my biggest challenges is keeping proper orientation of the craft at distance and in bad light conditions. The first thing I bought for solo was high powered light system and so glad I did. Some of the options include $29.95 Polar Pro kit which plugs into solo power, lights with their own batteries, or custom kit like the one recently advertised by MDog in the Classifieds.