In summer...
...it's time to swim in the lake.
There are several public beaches in Constance. Most of them are for free. The biggest public beach is called "Hörnle". It's where most inhabitants of Constance go to in summer. The best way, to go there is by bike. The bus is an alternative (number 5). To go by car is not reommendable. On hot summer days the lake cools down the temperature in Constance a bit. Then, it is less hot, than in other Southern German towns (Article about the climate at Lake Constance).
...mosquitos annoy people.
To sit at the shore of the lake in the evening could be just great, if there where no mosquitos. So, in the evening, take long trousers and a jacket (which is not too thin) with you, even if it's warm. During the day, mosquitos are not really a problem. And in some years there are considerably less mosquitos than in other years. So, you may be lucky and mosquitos won't annoy you too much.
...the inhabitants of Constance go by bike.
In summer, even people who actually love their car, ride on their bycicles. There are good bike paths in Constance, the landscape is beautiful. Often, the bike paths are much closer to the lake, than the roads, where you can drive with a car. At Lake Constance the bike is the best means of transport. It's the best way, to get to know the landscape. And at least in summer it's real good fun to ride your bike next to the lake. (Article about the bycicle tour around Lake Constance).
...excursions on ships are fun.
On the excursion ships on Lake Constance, you can take your bike with you, as well as on the Catamaran from Constance to Friedrichshafen and the ferry from Constance to Meersburg. Tickets can be bought on the ships, with the exception of the Catamaran (there is a ticket machine at the quay). There is a number of interesting destinations for an excursion from Constance, where you can go by ship.