This is probably a purchase you need to do together.

However you probably want a surprise factor for Xmas day right?

Here's some suggestions-

Make a gift voucher
Purchase a mini bicycle (model, jewellery or similar) with a note attached
A Bike bell or bike horn
Get a friendly child to draw a bike and frame it with a note

If I unwrapped any of these and it said "hey we are going bike shopping" I would be totally excited.