1. Clean your bike. You can start by just rinsing your bike off with the hose. No need to use soap. Just wipe down everything, including frame, spokes, derailleur, pedals, etc. with a soft rag.
2. Check your wheels. Spin them around. If they are rubbing anywhere, or make unusual noise, call your favorite bike shop.3. Inspect your tires. Don't assume the flat you have is just from storing all winter. Look for cracks in the sidewalls, which is easier to do if the tire is not totally inflated. Check the stem (is it bent or broken?) This one gets me every time.
4. Reset your mileage or note starting mileage, if it helps to have a starting point. Also, this may be a good time to put a new battery in the odometer.
5. Check your cycling wardrobe. You don't want to notice the day of a ride your old jersey has a huge grease spot on the front or your elastic on your shorts are shredding, or worse -- the back side is getting a little thin. Yikes!