Paddle board is a silly gimmick. I gave paddle board a 15 min try-out and reckoned it was better to stick to things with sails attached.
As Kloeten said, if you want a work-out, swim. When the water is totally flat or with minor little wavelets it is fairly easy and not at all strenuous. Once there are tiny waves (think 10cm-30cm) then it is going to be very difficult. When you start looking at classic waves rolling onto a beach from the ocean you are into surfing with a useless paddle getting in the way.
How about a kayak? You can get inflatable ones that are quite useable both on lakes and the sea. However, again don't underestimate the problem of getting back on board if there are waves. Mountain lakes can be very attractive but for water sports they need to be treated with the utmost respect. You get thermal effects in mountains and a flat lake can get very choppy very quickly when some sort of thermal wind kicks in.