The castle of Chaumont is in a beautiful setting: with the Loire River below, and a magnificent park, plus a festival of gardens which you enter (around 9 euros for the Castle, 10 for the festival of gardens, 15 for a combined ticket). Chaumont castle is interesting for its history and architecture. It changed hands from a count to the king, and Queen Catherine de Medicis, before ending up being owned by Prince in the 19th century. An interesting site (although I always think that if you have already visited several castles the interiors always end up looking the same - so if you have already made some visits to castles you may be happy to settle for the parks and gardens).
Chaumont Castle has extensive gardens, like many castles in the Loire area. These are divided into two spaces: the castle park itself, and the space for the Garden Festival, which is open to many artists, who are challenged each year to create something more spectacular than the last year (there is a different theme each year). Admission to the festival is paid separately from castle entrance: it's 10 euros to enter the festival space (complete entrance, which includes the festival, the castle and its stables is € 15.50, and 9.50 € for just the castle). I'm not a fan of nature, but walking through the Festival (which is open from late April to mid-October each year) was a delight: between amusement and amazement, I discovered original creations, combining flowers, trees, plants for the 2011 theme, "Biodiversity gardens and happy future." If you don't want to rush, I recommend a couple of hours for the festival, and another half day to visit the castle or stroll along the banks of the Loire River .. A beautiful day in the Touraine!