Depends on what speed you want to be able to go.
For a maximum speed of 35 mph a 50cc scooter will suffice. The Yamaha C3 has the lockable storage of a small car. This might work back East somewhere, but here on the Left Coast only being able to do 35 will kill you.
Next up, speeds to 50 mph, 125-150cc bikes (which includes scooters.)
If you need to get onto the highway you will want to be able to hit 70 mph, which means 250cc. If the highway use will be many miles and/or speeds will get above 75 (So.Cal. Freeways!) you will need a hot 250 (Ninja or equivalent) or 400+cc.
Scooters are handy with a twist and go auto transmission and splash protection for riding on wet streets and lockable storage. Motorcycles have a better image. Cruisers get to sport saddlebags for storage.