As the largest lake in England, Windermere is a huge tourist attraction. As well as offering watersports from sailing to waterskiing (and being unique in the Lake Disctrict to allow waterskiing), Windermere's central location makes it an ideal base from which to explore the rest of the Lake District. It is served by three major towns, Ambleside at the north end, Bowness approximately 4 miles south of Ambleside and Lakeside at the extreme south. The town of Windermere itself is set back slightly from the lake and is a short walk north from Bowness. The lake is almost 10 miles in length and at its widest point is about 1 mile wide.
The lake is surrounded by some of the most stunning countryside in the UK, I recommend doing as many local walks as possible, Ambleside to Bowness is short enough to be accomplished in an afternoon and takes the high route over the fells, offering stunning views down the lake. Bowness to Lakeside is a slightly longer route, though flatter, taking in the Windermere chain ferry (which is worth the visit on its own. Bowness to Hawkshead goes over the fells to the picturesque village of Hawkshead, it is fairly challenging, with a couple of stiff climbs but the views over both Windermere and Esthwaite Water are worth it (as are the pubs in Hawkshead).
If you want something slightly more relaxed, the best way to see the lake is from a boat. There are boat hire facilities at each of the major towns, but if you want something more complicated than a rowing boat or motor boat, you are better off in Bowness where sailing boats, catamarans and motor cruisers are available to the more adventurous mariner. The lake is well marked with yellow buoys marking deep channels and red buoys to mark shallows.
If you are looking for somewhere to stay, there are many many hotels and guest houses, from the large five star hotels in Bowness and Ambleside to the countless family run guesthouses that cluster around the lake. There are also excellent caravan parks, particularly Fallbarrow, in Bowness.
As with most of the Lake District, most of the local attractions are owned and run by the National Trust. Including two of the most picturesque picnic spots at Fellfoot Park (At the extreme south of the lake) and Brockhole National Park Centre (approx 1 mile north of Bowness). Fellfoot is an ideal place to take small children as the lake has narrowed to 50 metres or so at this point and is 5 or six feet deep at its middle point. This makes it ideal for swimming, especially as the shallow water is warmed in the summer making it a remarkable pleasant experiance. There are lots of climbable trees and a huge area of lawn for playing games. The park is served by the ubiquitous National Trust coffee and gift shops. Brockhole is the estate of an old country house, now turned into a National Park Centre by the National Trust. There are extensive gardens (for parents), an Adventure playgound (for kids) and the usual range of National Trust money making ploys (gift shop, etc). What makes Brockhole unique is that there is always something going on. Windsurfing, kayaking, dry stone walling, nature trails, falconry - almost everything is free and it is run by extremely competant professionals.
There is not enough room here to go into why I love Windermere, just buy Swallows and Amazons, rent a small sailing boat and go exploring. If you see someone careering about recklessly in a Hunter 22, give me a wave :)