A triathlon specific watch
A triathlon specific watch. There are a few out there now. The brand leaders seem to be Garmin and Suunto. Chances are if you want to buy this sort of gadget someone at the club has one or had one. Most multipart watches also come with a heart rate monitor, you can use this when you are running, cycling and swimming (with the correct bits). Ask at a training session or post on Facebook, members are only too keen to share their knowledge of new bits of bling!
A turbo trainer is great for those winter days when you can't get out onto the road. If you attend the club turbo sessions on a Thursday we have some you can borrow but you can't take it home with you. There are many different types on the market, it all depends on your budget...
Speed and Cadence sensors
These are essential if you want to monitor your speed and cadence. They will pair with any triathlon specific watch or dedicated cycle computer. Some require magnets attached to your frame, I would recommend the garmin ones as they don't require any magnets and are really easy to set up and use.