Most shops will offer 10-20% off all accessories purchased with a bike, so it's best to buy them then if you can. May I suggest...

Helmet
Seat bag
Spare tubes
Patch kit
Tire levers
Floor pump
Frame pump
Multi-tool

and maybe...
Moisture wicking top
Shorts with padding (don't wear underwear with these)
Better pedals and matching shoes
Lights if you intend on doing night rides