Arboretum & Botanic Garden, Pasadena
If I posted this on a City of Arcadia page - no one would see it. Arcadia is 5-10 minutes (depending on traffic and how you drive) east of Pasadena.
You can enjoy seeing 127 acres of botanical gardens. Hours are 9am - 4:30pm Monday thru Friday and every other Saturday and Sunday.
Also, during the summer, the California Phil Harmonic does outdoor concerts at the Arboretum. I haven't done this yet but look forward to doing this if I'm in town. You can bring your own picnic or order food there. Here's the link for concert info: