A lock. Many schools of thought will be proposed on this subject regarding needs and configuration with many compelling arguments. Before the shouting begins I would advise that whatever system you do not skimp in any fashion in this area, always buy something with a Sold Secure certificate and that whatever you use meets the conditions of your insurance.

Spare inner. Get two of them, you'll use them sooner or later.

Tire levers. Don't buy metal and be prepared to try a few until you find a type that's right for you.

Puncture repair kit. If you're tires are over 60psi then do not use self-adhesive as they have a tendancy to leak. Vulcanising solution patches will last longer then the seams of the tire if applied properly.

Hand-pump. Topeak Morph is the pump du jour at the moment. Worth the money if you're likely to get caught more than 10 miles from home or a friendly shop. Otherwise a mini will suffice. CO2 Canisters will get recommended but cost vs. value is to be thought about before buying.

Track-pump. If you don't have a Morph or similar, get one of these. BETO, Joe Blow are two good ones.

Minitool. Should at least have all the allen keys you need, a phillips and a stanley screwdriver.