First, check the website of the yard you are planning to visit. All the ones around us have their pricing sheets listed on the website. I print them out and carry them with me, so that when I'm in the yard, if I find something I can reference price immediately (or book mark their webpage on your phone).
Like mentioned, bring some dollar bills. Most yards will charge $2.00 to get in. All the yards around me have either carts or wheelbarrows on the yard, so unless they are all being used, you shouldn't have an issue with that.
I usually stock my bag with a standard socket set, a set of screwdrivers ranging from stubby to 18 inches, a set of lock tights, a paint scraper, a wire cutter, a set of gloves, a steel pipe breaker bar, and an 18" ratchet with appropriate adapter.
Then, the night before I go, if I know what I am looking for, I will check my XJ in the garage and see if there might be any specific items I need to pull my part - maybe a torx driver, maybe an allen wrench, maybe a deep socket head, maybe a tiny ratchet with shallow socket heads, maybe a crowbar.
If it has been raining, I will usually carry a tarp, or a large sheet of plastic to lay down on. Depending on what you are pulling, you may want to bring a towel also. However, if my hands get messy, I usually just find a mini-van and wipe them on the seat.