There are loads of pubs in Keswick. Quite a few of them belong to the local Jennings chain, and these are my favourites. The others are worth a look too though - most of Keswick's pubs are quite good and serve good beers and decent food.
At busy times of the year, and especially Friday & Saturday night, they can get rather full, sometimes uncomfortably so. At those times you can try the quieter hotel bars rather than the pubs.These still generally have good beers, and the atmosphere is not *too* formal (but moreso than the pubs maybe).
There's a lot more details about Keswick pubs on my Keswick page.
Dress Code: Anything goes, generally.
The Theatre By The Lake is a nice place to go and see a production. usually the plays run in "seasons", 3, 4 or even 5 things running simultaneously and performed on certain days of certain weeks. You need to check ahead to make sure you can see what you wnat on the days you are there (use the website). There is also a smaller auditorium called "The Studio". Both this and the main house host post-performance "discussions" for real theatre buffs!
The theatre is in a lovely modern building that is far, far removed from it's predecessor "The Blue Box Theatre" which was a load of blue caravans joined together to make one of the most unusual and unique theatres that can ever have existed. Performances in there in summer were almost unbearable as the sun blazed down on the metal skins of the caravans and made them into virtual ovens! I think there may be some memorobilia of that old theatre in the new one.
Dress Code: As nice as you like.
Keswick is easily the best place around Derwentwater for nightlife, though you could have a good night in a pub in some of the other villages.
Keswick has loads of pubs and loads of eating places of all types. And I've probably been in every one of them several times, though of course I have my favourites. I'll do more about Keswick's nightlife on its own page.
As an alternative form of "nightlife" you could just sit in Crow park and watch the sun go down :-)
Dress Code: Whatever.