There is no shortage of sports-related activities in Lausanne during the summer.  If you're really ambitious, you can run up and down the hills to train for the annual Lausanne Marathon.

Rollerblading on the Ouchy shore is popular weekend pastime; you can’t help but notice a few very talented rollerbladers who set up obstacles to dance their way through.  This pastime is so popular that the annual rollerblading competition is held at Ouchy.  Lausanne has a pretty active “alternative” scene, so you will see skateboarders and bikers showing off stunts as well.

There is a nice system of Bike Sharing, and you can find an iPhone app called bici which tells you where to find the closest available bike.

If you want to go for a swim, the public beach/pool is straight down Quai d’Ouchy if you turn right from the Metro line.  Buses from Place St. Francois may be more convenient and save you the walk.  You have to pay a few francs to get in, plus locker rental fees, but this is the only part of Lausanne that actually has a sandy beach with lifeguards in addition to the two pools.  Near that same area, you will find a number of volleyball and tennis courts.  The an annual beach volleyball competition is also held in Lausanne.

If you continue along the shore, you will pass the sailing center and eventually get to a park area where locals like to have summer barbecues and play games of Frisbee.  

Lausanne is also conveniently located for anyone wanting to go skiing in the winter.  Villars, Leysin, and Les Diablerets are all manageable daytrips from Lausanne.