I love the Botanic Gardens. They're beautiful, and perfect for either a romantic date or a family outing. My first visit was in February, and I thought that it wouldn't be so pretty since most of the flowers wouldn't be blooming. I was wrong. Though some of the plants had died back, there were many, many still in bloom--and the architecture and landscaping was enthralling. I've since visited it in spring and summer, and I always enjoy myself. There are rose gardens, wildflower gardens, picturesque pools and fountains, and a wooded area with a pond with turtles and fish--and all of it for free.
Well, there are some areas that aren't free. The Japanese gardens are definitely worth visiting at least once, even though you have to pay to get in. Kids will love the giant, very friendly coy (they practically crawl out of the water to be fed these little pellets you can buy a quarter to get). There an indoor rain-forest garden, which I don't think I ever visited.
One word of warning--watch out for brides, especially in springtime! This is a favorite spot both for weddings and wedding photographers. Thats why I prefer going mid-week or off season.