Can I implore you to use public transport as much as possible. Particularly if you want to walk up Catbells. There's an excellent bus service from Keswick that drops you right at the cattle grid at the start of the main walk up.
The Lake District is clogged with lazy drivers - the bus service in the summer season is excellent - there are timetables everywhere and you don't have to worry about parking. Something has to stop the moving traffic jam that is the road from Windermere to Ambleside!
And a local tip: if you want to do Catbells, start at the Borrowdale end of the whole ridge. Get the bus to Rosthwaite and climb up to High Spy, then you have a fabulous high ridge walk via Maiden Moor to Catbells, and you can walk down (passing all the grunting people who thought Catbells was not steep!) and catch either a bus, or a ferry back to Keswick. But Catbells straight up from Portinscale is steep. And awfully crowded.
It's quite strenuous if you're not used to walking. For novice fell walkers around Keswick, Latrigg is a much lower fell, and far less crowded. Or try walking from Rosthwaite to Watendlath, where there's a nice place for afternoon tea.
Langstrath valley - again from Rosthwaite in Borrowdale - is pretty flat & very beautiful, with swimming holes for a hot day. Although this summer it was ruined by scum who left tents, rubbish, and scarred turf from illegal fires.
Old Man is again a major fell and if you're not used to fell walking or navigating stick to the tourist path - but it's hard going and probably the most boring way to get up to the cairn. Alternatively, try Tarn Hows instead, or catch the steam gondola across Coniston Water to Brantwood, a really interesting historic house, and have the cake at Spinning Jenny's. Lots of walks into Grizedale forest from there, or walk through to Hawkshead which used to be the perfect Cumbrian village (a bit too much tourist crap for my liking nowadays), but St Michaels and All Angels church is glorious and the school Wordsworth attended also worth a look.
Cockermouth is a lovely town outside of the Park, but full of interesting independent shops, and Wordsworth birthplace is worth a look round. As are Rydal House and Dove Cottage. The area around Grasmere and Rydalmere is full of interesting low-level walks eg the coffin track between Grasmere and Ambleside or just following your nose on Loughrigg Fell - it's not strenuous. The walk from Grasmere to Easedale Tarn is not too strenuous and you can go on from the tarn into Far Easedale or even up & over into Borrowdale - but that is a full day, strenuous walk needing a map.