It can't just be hiking and biking all the time. Like today, with temperatures around 30 degrees Celsius (86 Fahrenheit) and a humidity similar to a steam room, we suggest a less strenous activity. How about renting a kayak or canoe on Lake Thun. Your ideal starting point could be Neuhaus camp site near Interlaken. Other activities such as wakeboarding, wind surfing, paragliding are also available.
Oh and one other thing. Once on the lake with your kayak, watch out for the larger passenger boats. Their wakes are fun but don't try to demonstrate that you are a superhero.