You could spend months tramping the hills, valleys, and fells of the Lake District, or, in three days, you could drive through the major towns and villages. The key is not to do too much in too short a time. If you are traveling by public transportation, many places will be off-limits.
If you must tour both south and north lakes together in a short time, start in Kendal. After you've looked around the market town, move on to Windermere and Bowness-on-Windermere, where you spend the first night. You can take a boat trip on the lake that afternoon up to Ambleside. Next day, cross Windermere by ferry, and drive through Hawkshead and Coniston to rural Elterwater, where you can have lunch in one of the fine walkers' inns thereabouts. Spend the afternoon in Grasmere and nearby Rydal touring the sites associated with Wordsworth, such as Rydal Mount and Dove Cottage. Your second night is in Keswick, and on the third day, you can loop around Derwentwater through Borrowdale and isolated Seatoller to Cockermouth, Wordsworth's birthplace. From there it's an easy drive east to the market town of Penrith and the M6 motorway, or north to Carlisle.
Kendal, in the southern part of the Lake District, marks the starting point, followed by a drive to Windermere and Bowness-on-Windermere and a cruise on the lake that afternoon up to Ambleside. The next morning you can mosey around the shops and museums in Bowness before venturing on to the Lake District National Park Visitor Centre at Brockhole. In the afternoon, cross Windermere by ferry, stopping in Hawkshead and Coniston, before ending up at Elterwater. This is a splendid place to spend the night in peaceful rural surroundings, and you can take in one of the local walks the next morning. Lunch and your overnight can be in Grasmere, just a short distance away, giving you plenty of opportunity to explore that lovely village. From Grasmere, Keswick is the next overnight stop, allowing you to make a day trip into the gorgeous Borrowdale Valley and perhaps take a boat trip on Derwentwater. On the final day, you can see Cockermouth and Penrith.