Swimming from a boat in the Blue Grotto during a cruise around the Bay of Kotor. You wouldn't be allowed to do that kind of thing during a boat trip on Windermere!
As Montenegro is not (yet) in the EU, double-check the situation regarding data roaming.
If you plan to tack on Dubrovnik at the end of the holiday, as I did, consider staying two nights rather than one. That gives you more time to arrange things like a kayaking trip or just to go swimming. As none... (more)Louisa Radice