The woods around there are pretty awesome, some fab light this time of year come early evening when the sun is going down.
There are quite a few grand hotels, The Golf springs to mind and there are others on the same road. They have some beautiful gardens. One of them has a great long archway of what I think were Hydrangeas, it could have been The Golf but I can't find reference to it online so it could be one of the others. Now when that is in bloom it's amazing.
Also you have Tattershall just down the road which sports a lovely castle, and some great lakes nearby in the Tattershall Country Park. Lots of thing going on there, water sports, fishing, golf, and then there are some nice little nature areas. If you hit it just right time wise you will see a mass hatching of damsel flies. We witnessed it last year and it truly was an amazing sight!
Then there is Lincoln not too far away. Lincoln cathedral is nice. In a built up area so the outside isn't as appealing as some cathedrals, but the inside is spectacular. There is also a great big nature reserve in Lincoln (well just outside the city centre).
Or even Boston
You have The Boston Stump, which is again amazing inside (but I would say that!). The market town and ... erm there are lots of other things but my mind has gone blank
HTH