The botanical gardens are a great place for so many reasons. There are amazing events there quite often. Ballet, concerts, plays, etc. Try to catch Chihuli in the gardens if you can.
Definitely try to go in one of the cooler months, it's obviously pretty hot in the summer.
It's open quite late though so you can go closer to sunset even in the fall or late spring. It's a great place for a stroll, a picnic, a date.
There are sometimes free passes through the library system as well. If you can grab one of those then what are you waiting for!
Great for kids as well. They'll enjoy the gardens and there's usually a bit of wildlife as well! So put on some suntan lotion and get out there!
Ps: for real, chihuli in the gardens was one of the most beautiful exhibits I've ever seen. Put that on your calendar.