I carry a small first aid kit, a lock, and a wallet.

In all the years of commuting (about 16 years) I have only had 1 flat so I locked the bike at a cafe (I asked first) and caught the train home. Came back in the afternoon and fixed the tube and had a coffee. No worries.

The first aid kit I've used on many occations. Once on myself and the other times on other people.