We spent last week-end at the Château de Versailles for my husband's birthday. It was a cold day for a walk in the gardens, but beautiful nonetheless. You don't need a ticket for entry and you can definitely spend an entire day there. It's the perfect place to go running or cycling and visit with the kids. Sheep graze in the fields and swans swim in the large bassins. In springtime you can rent bikes and row boats. There are hot potato stands and a few cafés on the grounds - even an Angelina, although I think a picnic would be best. I also wrote about Versailles here and here.