Also without going overboard on buying stuff, a pair of cycling gloves, can be bought pretty cheaply and makes a vast amount of difference to the comfort of your ride. I cannot believe I cycled 3 years without any.

My basics would be:

1)Helmet, lights + lock
2)pack
3)gloves
4)semi/slick tires
5)bottle + cage if going a distance
6)padded shorts
7)basic tool kit, spare tubes, tire levers
8)helmet mirror
9)a light waterproof jacket,
10) something glaringly obvious that I've missed. (chain lube as well)

A pump of course!