A travel site offered a local deal for the Sunken Gardens of a discounted annual membership for the family. Not much more expensive than the cost of individuals tickets for our family of three to visit once, we went for the deal.
An added benefit was that the voucher could be redeemed up to 6 months after the online purchase date and the membership itself would be good for a year from the date of redemption, not the online purchase date. This gave us a sense of comfort, since the Gardens are in St. Petersburg, and we were not entirely sure how soon we would make a visit that way that would include the Gardens. We pushed the benefit to limit, redeeming our voucher just days before its expiration.
The day of our redemption was rainy to the point that the Gardens were closed for the inclement weather! Fortunately, the desk was still manned so that we could redeem our voucher and establish our membership. Now we have a membership, but still have never visited the Gardens, which we have only ever heard are an amazing must see.
Not to disappoint our son, however, we needed something to do despite the weather and there where we were. The Great Explorations Children's Museum of St. Petersburg shares a building and entrance with the Gardens. Hank loved the museum. He even got to visit the "beach" and play in sand with beach toys on the rainy day. (Our original plan had been to visit the Gardens and St. Pete beach.)
I recommend Great Explorations as a place a preschooler with thoroughly enjoy, but the admission price is high. Definitely consider a membership if it is a place you will be able to visit often and/or you can use the reciprocal membership frequently.