B.C.
One color or mark to indicate removal.
Have sunscreen on hand.
You can get some bandanas to hand out as party favors - they are great sweat rags and if people wet them before tying on their heads they can help keep them cool.
http://www.orientaltrading.com/western-bandanna-assortmen...
http://www.orientaltrading.com/neon-bandannas-a2-15_17-12...
Have a dumpster to collect debris.
People will need a way to wash up from working before lunch or dinner.
The nice thing about pizza is you can have it delivered so you don't have to do the cooking and clean up is easy.
Other picnic foods or KFC or Boston Market works well, too.