Naples Botanical Garden in Naples, Florida. For free. Yes, free.
If you go to bhg.com (Better Homes and Gardens) you too can get as many tickets as you want/need to visit a local public garden for free this Friday too.
Bok Tower Gardens in Lake Wales is also honoring this free day. I went last summer, for a free museum admission day. If you have the chance to visit there tomorrow, or ever, do it. It's a magical place.
If you happen to go to the Naples Botanical Garden with your free tickets look for me. I'll be the middle aged woman looking like a kid in a candy store - I'm pretty sure I'm more excited about this opportunity than the kids are (though one asked me at 8:00 this morning if it was time to go to the garden, so maybe we're all equally excited).