swimming is a one of the best work out for activities. Because people have such good systems is because of the energy of the work out that diving can give you and as a result it can really help with all over body health and health and fitness.
Swimming or cycling I would say. Especially when abroad and you can do it in some beautiful scenery. Diving of course, both scuba and for pearls (I was once lucky enough to be taken on a pearl diving trip once, diving for pears like these at mesenso. Bloody hard work, but magical). Or squash for cardio - and that's the most fun!