Lots of good advice here, I think you'll be fine at Windsor castle, you'll out of tourist season so should miss the queues and the chapel is well worth seeing. I would then walk through the park to the statue (will take about 40 mins return) and have lunch by the river. There is a nice pub/restaurant just across the bridge on the Eton side of the river. Not much else to see at Eton though you can walk down to the school, will take you about five minutes.

Fat Duck and Waterside in Bray are both excellent, though check the pricing and book before you set off. I use these guys http://www.geminicars.co.uk/ for heathrow pickups when not using my local firm.