Just returned from Ambleside yesterday. Nice ride around from there to Wray Castle and across the ferry to Windermere.
Ambleside to Troutbeck was a nice route.
Can also recommend Whinlatter near Keswick and the Borrowdale Bash route.
If you are in to road biking, there is also some very nice road bikes in Wheelbase including an all Disc brake Colagno for 10k (one of only 5 in the UK apparently) and a red Colagno Master full build for 4k.
Push bikes in Ambleside also have a very nice Baum frame and fork for 4k ish – very tempted.