Depending on the distance of the geocache away from your hub, you may want to pack a little something extra for your group members.
If you're going to be out for more than few hours, bringing enough water can be important, especially in hot climates. Frequent geocachers might want to invest in plastic or metal water bottles, just to cut down on their trash.
Don't be afraid to make a day of it: bringing a snack might be nice and all, but it's hard to beat a full-on picnic if the geocache is in a pretty location. This also means you can be out for a longer period of time, hitting up multiple caches, before heading back home from the hunt.
Of course, you need to attend to the geocache itself. Making a trip to the dollar store before heading out can provide you with some cheap, cute trade items for the cache. Make sure they'll fit, though - check the size of the cache before purchasing items. It's hard to fit an “I Love New York" coffee mug into a microcache, after all.
If you're not too keen on dealing with brambles or broken glass, or even a bit of dirt, a pair of gardening gloves can go a long way as well. Bandaids, Ibuprofen, alcohol wipes, tweezers, and some other basic health supplies might also come in handy. You never know when someone might step in a bee's nest or get a sliver!