I'd probably head into Eton to eat, but there are plenty of coffee shops and restaurants to choose from.
There are some quirky shops in Eton, so have a look round.
I live too close to eat there really, but if I had to choose, I'd go to Al-Fassia, but I've not been for years. I sometimes eat at the King and Castle as it is cheap and has an outside balcony area
The Christopher Wren hotel food is nice, and there's Brown's - both have a river view.
A bit further out, there's a Harvester in Old Windsor that's by the river. Old Windsor is about 3 miles from Windsor.
If you look on eating out guides, many places will be listed as Windsor, but they'll be miles from the castle.
The river trip is worth doing. Best view of the castle.
The Long Walk is long but nice
Saville Garden is good.
You can take a tour bus, but I've not been on it. It probably covers Runnymede.
The Theatre Royal is lovely and puts on some good plays.
There's also the Old Fire Station.
For a cinema, you'll need to go to Maidenhead.
Egham used to be delightful but the little shops disappeared after the supermarkets opened. Has a few nice charity shops.
Slough charity shops are great, but you'll need to rummage. Slough also has a lot of world food type shops. Shopwise, there's not much there other than TK Maxx and Primark. You can walk from Windsor to Slough. It's about 2 miles on foot, and flat, but you can also get a train.
Watch where you are staying. If the hotel is the Slough-Windsor Hotelname, it will be in Slough.
If I wanted to head out somewhere, I'd look at Cookham for the Stanley Spencer gallery, Marlow or Henley for riverside. Otherwise, I'd head towards Richmond and Hampton Court.
If you want fancy meals out, there's the Waterside and the Fat Duck in Bray, but that's just outside Maidenhead. I've not been, so can't recommend them.
If you have DC who are under about 8 yrs old, there's Legoland, but if older there's Thorpe Park or Chessington and neither are very far.