Lakes are popular but don't really get warm until July. The outdoors is generally bug-free so sunbathing can be enjoyed in peace. The lakes around Sallanches offer good fishing, swimming and windsurfing. Lake Passy is especially popular for watersports.
Rafting is a good "white knuckle ride" and the Arve (here) and the Giffre (at Samoens) are popular local rivers. The Arve is cold! Further afield are the excellent Dranse (between Morzine and Lake Leman) and, in Italy, the terrifying Dora Baltea. If you can swim - give it a go.
Either as a short 2 hour run or a full-day descent, complete with a picnic on a sandbank. It's a great, exciting, memorable experience. There are a wide variety of rafts, from the ubiquitous zodiac to one or two person inflatable kayaks. The smaller craft are more vulnerable.
Find out about local activity companies. They offer a range of watery adventures from 2 hrs to several days and for both beginners and experts, eg: Rafting intro, Swimming the Arveyron, Water safety courses, Multi-activity adventures. Companies also offer rafting and kayaking on all the major rivers in the area.
Known as 'riverboard' to North Americans and 'white-water sledging' to some below the equator, hydrospeed (white water swimming) is offered by all the rafting companies and is great fun.
Thick, reinforced wetsuits, crash hats , flippers and a block of plastic held in front for buoyancy and protection are provided but you'll still feel vulnerable.
Kayaking around here is good but not well documented. Ask us about possible sites or chat to the ever-helpful rafting companies. The Giffre is good and grade 3 (below the grade 5 Sixt gorge). The Dranse above Thonon offers a range of levels but ring the EDF for release times and volumes. The Dora Baltea features a range of difficulties including 15km of grade 6!
Buy the Swiss Canoe Club river map which grades rivers around here and throughout N. Italy including the Gran Paradiso.