You're going to want a stage of sorts, even if it's plywood sheets on top of pallets.
Who's supplying power? Make sure everything is well grounded! (Another reason not to be in contact with the dirt.) Google Leslie Harvey.
Some sort of protection for both the band, and the PA stuff is a great idea. You'll want to stay dry if it rains, and you'll probably want shade if it's sunny. Also be aware that the wind can blow your bass (or guitars) over if they are sitting on stands. (I usually tuck mine in back on the lee side of my amp to minimize this risk)
5K run sounds like there may be children in attendance. Children do not seem to have respect for your stuff, and have no concept of boundaries. If there are kids running around, keep an extra close eye on your stuff!