It depends on what you want to do with the bike.

After helmet, my list of firsts would be:

Pedals (if that bike, like most, comes with cheap, plastic ones)
Shoes if you want clipless pedals
Bottle cage and bottle
Pump (whatever kind you want)
Patch kit
Spare tube (may as well get a couple at least)
Floor pump (though you can live without it if your have hybrid wheels that use automobile valves)
Little cycling toolkit (even if just to tighten things that come loose)
Saddle bag of some kind (the under seat kind, get a larger one than you think you will need so you can stuff other things in it besides the tube and toolkit)


Nice to have:
- whether you end up wearing cycling clothes or not, it's very nice to have cycling gloves.
- some chain lube (I like ProLink)
- a bottle of Pedro's Bike Lust cleaner.

Anything else like lights, rear rack, etc. depends on what kind of riding you plan on doing.

Just my opinion, but if you get that discount, I would go ahead and just buy the biggest ticket item: clipless pedals and shoes. For that kind of bike, I don't think you could go wrong with Shimano M324 pedals (regular pedal on one side), and walkable shoes.