I can’t believe I only went here — the 385 acre Chicago Botanic Garden — one time. It has a rose garden, a Japanese garden, and MUCH more. You can see its wikipedia page for a good overview. If you’re in the area, you need to go!
Admission is free but apparently in 2015 they started charging $25 for parking. However, you save $5 if you buy online in advance. Twenty dollars isn’t bad at all split between 2+ people, especially given that you can easily spend hours and hours there.
PS. If you want to avoid parking entirely… I can’t verify this, but, according to an August 2015 Yelp review by Rosa J., there is pedestrian access into it, and a bus that goes right to front entrance. It’s the Pace #213 Bus to Northbrook Ct., from Davis St. Transportation Center. Just don’t confuse this bus with the other #213 Pace bus, to Highland park, she says, since that won’t work.