The first game we played for our Christmas in July playgroup (after letting the kids run around for a while) was Christmas Kisses. It's played like Hot Potato except that when the Christmas music stops everybody blows kisses at the person holding the presents. It's actually supposed to be played with Mistletoe but I couldn't find any in July, imagine that!
Next, we played Bingo. I found a website (listed on my sidebar) that does custom-made Bingo cards! Easy and fun, what else can you ask for when planning something for a big group of kids?? I didn't have prizes for, we just played until everyone got a chance to call out, "Bingo!" so that nobody felt left out.
And last but not least, we played Decorate the Tree. I made a big Christmas tree out of green and brown poster board and outlined the summer "ornaments" on the tree. Then, each child took a turn and put the ornament where it was supposed to go!