The park is in Fukushima, but its a short drive from Tochigi and well worth the effort. You can combine a trip to this park with a trip to Komine Castle (which also has an excellent rose garden).
There are two main parts to the park both of which are fairly quick visits. The gardens are a classic example of a Japanese garden complete with tea rooms. Entry to the gardens is 350 yen and for an extra 500 yen you can go to the tea rooms and have traditional tea (without the ceremony) in a pretty little tea house. The tea is well worth the price, especially if you are visiting Japan or haven't experienced ocha before.