The very first thing I do if it's a vacation near any sort of beach - even at the north sea- is drop my bags and go down to the water. If it's warm I'll take time to change into a bathing suit - if it's cold and rainy- no matter- I put on a raincoat and make a beeline straight for the sea.
If I'm near the water - I'll swim every day. I guess I have to say that's my very favorite travel activity - submerging in a warm ocean and just floating looking at the sky.
A lot of it depends on who you're with. Sometimes it's fun just to hang out and be lazy.
If the point is to explore and soak up the culture, I'll spend part of every day doing that - but I don't like those rush, rush, rush, see this and see that sort of holidays. I really love the ones where the place you're staying is full of people and atmosphere that are just as interesting as the touristy sights to see. I like eating lunch out - but cooking in at dinner- so I love holidays where you have a cottage or house you can cook in and maybe a local pub or bar to walk down to in the evening for a drink.