It depends on your level of motivation.
If you need competition to keep you interested, and need a sport, I'd be looking at something like Ultimate Frisbee depending on if you have local leagues.
If you can stay motivated without doing a sport, I'd do a tonne of running. Sprint work, join your local parkrun each Saturday, maybe enter a 10k or Half Marathon. Running has to be the best of the triathlon options because it abuses your body more, and teaches it to overcome injury and pain better. Cycling and swimming seem isolated and soft in comparison, and I just feel like when you get an injury or two during the season your body takes it worse.