I visited the UIUC Japan House multiple times during my UIUC tenure. It was always a premiere spot.
The Japan House + Arboretum is very close to easily accessible bus stops and is also in walking distance from campus.
During the fall, it is a good place for walking. There is a big willow tree that looks very nice.
The park is good for picnic-ing. I've done it before.
Cherry blossoms 1
During the spring, the cherry blossoms look great. I've included two pictures I took here. When the cherry blossoms are in full bloom, the spot is incredibly busy, with many students looking to get their glamorous photos. It is a fun activity.
Cherry blossoms 2
The Japan House has a rich UIUC history, both of cultural value and professorship. One can learn more online at many resources, or at the Krannert Art Museum.
The Arboretum is additionally great, with a very nicely shaded boardwalk and very pretty flowers. It's right next to the "Idea Garden".