Mercer Arboretum & Botanic Gardens is a neat place to visit, especially with family. The ladies will love it. It is a fairly small complex of gardens, ponds and sitting areas. Lots of beautiful plants, flowers and trees. It is a photographer's dream. I recommend you visit in the Spring when the flowers bloom. The complex sits adjacent to (south of) Cypress Creek.
There are two dirt trails that lead out of the main complex in opposite directions to other areas. The trails are all flat and easy to walk. At one end, there is a really nice pond and bamboo tree park. Sadly, they had some fake, plastic, painted ducks floating in the pond... c'mon!
Although tranquil, it does not provide much solitude. It sits off a busy road (Aldine-Westfield) and is close to Bush Intercontinental Airport. Still, it would be a nice place to sit, relax or spend time with a romantic partner.
If you love plants, this is the place for you. If you are an avid hiker, it's not.