Use a Bingo card template and write in activities or experiences your students might have engaged in over the summer in each space. You'll want to make several different cards, then photocopy them. I use about eight original cards for a class of 25 - 30.
Give each student a card, and ask them to find fellow students who fit the description in the square. They then write that person's name in the square. I make a rule that they can only use one person for two squares so they have to circulate.
When someone gets a Bingo, give them a small treat. I usually let them play until we've had five or so winners. Afterwards, we have a discussion about what they might have learned about each other.
You can find blank templates online, or if you don't want to make your own cards, I have some available here: