The town of Windsor is easily reached by train in under an hour. A five minute walk from the train station in the center of town takes you to the gates of the castle. The public rooms of the castle are beautiful. Be sure to check out the ornate wood carvings in almost every room, some by the master, Grinling Gibbons.
Leave time to poke around Windsor itself. Highly recommend The Crooked House for tea! http://www.crooked-house.com/ And the train station has some nice shops and cafes, too. See if the ladies selling adorable bean bag animals in Liberty prints are still there! It's always nice to buy locally made souvenirs.