Someone already mentioned Snuba - which is an interesting choice. I enjoyed that, as you're not bogged down by weight of equipment - but you're also tied to a boat and the setup I had didn't give me access to see the air left up top so I was always a bit paranoid.

Most dive shops have "Discover Scuba" which is basically they give you some brief training, but then they go on a scuba dive with you to a shallow depth (6m I think?) and basically hold your hand the whole time.