We stayed in this castle (Sundbyholm Slott) for 4 days to celebrate my wife's birthday. Originally, we had only booked 3 nights, but because it was so nice, we extended for another day. The castle is situated in a beautiful landscape, next to the Lake Maelaren with a lovely beach and park right next to it. Unfortunately, you cannot stay in the castle itself, but they have either a "Herregard" or mansion house next to the castle or very nice cottages a few meters further away (most of them are semi-detached). All of them are very peaceful and well laid-out.
Most importantly, however, was the food at the castle: one evening, we got 2 small starters and 2 main dishes, all of them being a real treat! The other evening we got their 3 course meal, which was again excellent (combination of fish and beef). In addition, our package included a "smoergasbord" which was similar to an afternoon tea at Harrods and equally delicious.
We can whole-heartedly recommend this both for food and for the environment. It may not be cheap, but what else would you excpect when you can dine in a beautiful castle dating back to the 16th century?