Villages/towns like Oundle, Thrapston, Denford and Aldwincle are pretty and have nice restaurants, shops and cafe. (Thrapston in particular is undergoing a bit of a revival at the moment, with posh florists and gift shops and delis apparently opening up all over the place). Local Authority is trying to revive Kettering as well - this will take a while IMO, as it has all the appeal of Scunthorpe at the moment. I am waiting to see how their plans for a 'restaurant quarter' materialise. Although it has a great independent Italian restaurant we really love.
Some of the country pubs are great, and do lock ins.
Wicksteed Park in Kettering is a brilliant small theme park with lots of play equipment (fee to use) and a really rural atmosphere.
Stanwick Lakes is brilliant, as discussed in other posts. We go there a lot.
Northampton itself is a bit dull, could be anywhere in the Midlands, but the shoe museum is really interesting (free, just rebuilt and extended), and there are some nice restaurants there.
It's quite a relaxing place to spend time, but I do find I am popping into Cambridge and London for clothes and things when I spend a lot of time there (have a shared cottage on the Nene).