I usually ain't a big sucker for touristic places and lakes don't really make my heart beat faster…read moreeither, but le Lac d'Annecy is a huge exception.
This lake is just the most clear, pretty and awesome I've ever seen, especially with the stunning mountains in the back. There's a super romantic bridge, you can go for a walk or run next to it, sit in the grass or go swim or just read a book in the park while watching people walk by and the lake staying as pretty as it is. - Or you can also bike across the whole lake and visit the other cute villages nearby.
But, an absolute recommendation is to rent a small boat (you don't need a license for that!), drive to the middle of the lake and swim. It's not the warmest lake as it's a glacier lake, but just do it.