If the weather permits, you can bicycle to Arisia and then park your bicycle in the Westin's garage.
There is a room on the first level of the parking garage for parking bicycles.
When you arrive at the hotel the first time, temporarily park your bike near the front entrance and go to the Westin's Front Desk and request a card key for the bicycle room.
Once you have a key, carefully walk your bicycle down the ramp (in damp weather it can be slippery). At the bottom of the ramp, go around the gate and go straight across the floor to the far wall (in the 1D parking area). There's a door that says "Fan Room" (by the time you arrive, there may be a couple of signs saying "Bike Room" posted). Use the card key to gain entrance to the room. There are a dozen (or so) hooks to hang your bike from the wall, there's also a metal loop for locking your bike.