What should I do to my car if I'm leaving for 2-3 weeks?

Disconnecting the battery is not a bad plan. Minor power drains can kill a battery over an extended period. If you are going to have a friend start it up, they need to drive it for 15-20 minutes. When you crank a car for a short period, water builds up in the system and does not go away until the engine runs at a normal temp for about 5 minutes. Most likely you could leave it the way it is and it would be fine. Have a great trip.