If you want to build muscle doing something fun, then kayaking is definitely way ahead of everything else mentioned. Most things like BMX, climbing etc. aren't actually that good a way of building muscle, as you're not doing the sort of continuous regular working of the muscles – unless you're climbing really hard stuff you shouldn't actually be using your arms that much, and the guys doing harder stuff mostly get their muscles in the gym.
Rowing would be up there, but it's not a very accessible sport given the commitment needed by a typical rowing club, apart from using an ergo which can get almost as boring as doing weights in a gym. Swimming comes close, but IMHO is far more boring than kayaking as you only get to see the same bottom of the pool all the time (unless you do "wild" swimming, which is recommended, but also has accessibility issues). Also not quite as good at building muscle as kayaking as the peak forces are lower – top swimmers also get their physique in the gym, whilst I've got quite decent upper body development just from kayaking with almost no gym work at all.
I would suggest XC skiing / roller skiing given that's the main way I've maintained my upper body muscle whilst I've been doing less paddling, but that's not exactly very accessible either!
