This park is famous for its poppies, which are usually blooming in early May. Free.
You have to know that this park exists to be able to find it. It is quite well hidden. There is a little, vertical green sign (in Japanese) just west of Tsukuba Soai Hospital, on the same side of the road as the hospital. Turn south onto that small road and follow it as it winds around rice paddies. You will then see some more signs (also in Japanese) showing you the entrance to the park on the left (east). Follow that road a bit until you come to a parking lot.