a bike kit, which is basically a set of screw drivers for different parts of the cycle. thats useful for making minor adjustments yourself.
I think something that makes a huge difference is riding position. depending on how you intend to ride, there is an optimum position of the seat and the handle (whether you lean forward for speed or want to sit upright for a more laid back ride). Also the seat should be relatively high, so that your leg is almost fully extended when its most stretched (i.e. the pedal is in the lowest position).
also if ur offroading (or even on the road) its a good idea to invest in some mudflaps. and reflective clothing if you're cycling at night
This seems like a good guide for the positioning of the seat and also how big your cycle in terms of frame size should ideally be
http://www.totalbike.com/service/frame_size.php