Honestly, go to the library and get an old book on party games. You'll be astounded at things you've forgotten and these kids have never seen. I did this when my son was little, and neighbors with much younger kids have done them recently. So much fun! It's all about exercise and not about winning or having expensive props! Sack races, potato races, bean races -- just use old pillow cases or wooden spoons from the dollar store and some potatoes! They have a blast, and they run around, semi-learn to work together, and it's great! Everyone can do stuff at one time, vs. something like "Pin the Tail" which has a lot of kids waiting in line. Musical chairs is good if you have something for the kids to do who are "out" - sometimes letting them be the ones to stop the music is a good thing. But it's tough on the first one of 15, so use caution if you have a whole crew.
For quieter times or for kids who don't like to run around, there are memory games you can play (put a bunch of items on a tray, cover it, uncover it and let them look, then recover it or remove it. Since they are only 6 and don't write well, have them work in teams of 2 or 3 with an adult who writes down what they remember seeing. (No helping! It's just for the kids!) Use common household items: pencil, straw, spoon, paper clip, pieces of string, spare keys, salt shaker, small paper plate or napkin, toothbrush, mirror, hammer, roll of tape, bottle of glue, small toy like a hot wheel or lego, anything.)
You can also do a treasure hunt - we buy those plastic Easter eggs (and you can call them anything - dinosaur eggs, alien eggs, anything). Put little prizes or candies or stickers in them, then hide them in a room the kids aren't using. If it's an empty hall, bring in some things that can shield the eggs - a wastebasket, grocery bags with your supplies in them, a spare briefcase or laptop case, a pair of sneakers, you name it - and put the eggs inside. The game is the entertainment, and the stuff they find is their prize so you kind of give away the goody bags early on with no extra expense. You can split it up by giving each kid a color or by putting a sticker or symbol on each egg - then give each kid a card with a sticker on it or that symbol, and say, "Find the eggs with your sticker on them." They can either ignore the eggs that aren't theirs, or help each other (which is really fun and builds cooperation). If you have a lot of kids, they can work in teams. Keep a list of where you put stuff too - makes it easier to help at the end or make up for a kid who didn't find much.
You can also do a little in the healthy snack area with some veggies and dip (ranch dressing, humus), which also helps the kids who have food allergies.
Have a great time! Just do less than you think you need to, and any ideas you didn't use or activities you didn't get to can be saved for next year.