For years you've been requesting a summit, and only now that it's been approved are you trying to decide what to do at it? If you didn't know what you wanted to do, why were you asking for it? If you're trying to build team interaction, have you considered starting with something less ambitious?
How about weekly / monthly cross-training meetings? Have one team member speak on a subject the others don't know much about. Rotate the primary speaker for each meeting. Each person will learn more about what the others do, their individual strengths, and you can get an idea as to who is suited as a trainer.
Another option is the 'Lunch N Learn' format. Assign a team member to report on a new technology at a team meeting held over lunch. Ask for volunteers; you might be surprised. For a department the size of yours, you could get three pizzas or some take-out Chinese for under $30.
In both of these suggestions, remember to take your turn "in the barrel".
Save summits for laying out big project planning, or other times when you need to meet uninterrupted to discuss a subject in depth. Don't have one just to do 'team building' exercises; no one takes them seriously.