If Windsor then The Ivy is perhaps a safe bet for a place to eat, and very close to the Castle. Same would apply to London, but only if one is co-located to your attraction. Also recommend a Burger & Lobster (for the lobster rolls)

Second the Science Museum, has a lot of good exhibits on at the moment, need to buy tickets for a set entry time. Also the Wildlife Photographer of the Year exhibition at the Natural History Museum - museum entry is free, but WPotY requires tickets. Plenty of places to eat around there.

If she's into fashion/design then the V&A museum might be of interest.

If you're at the museums, or she would prefer shopping then Harrods & Harvey Nichols are further up the road, and there's a Burger & Lobster at the top of HN. Harrods Food Hall can be fun too.

The Food Hall at Whole Foods on Kensington High Street used to offer a good choice of cuisines, but haven't been for quite some while.

Market Halls food court on Oxford Street supposed to be good, not been yet.

Sky Garden has lovely views of London, but weather dependent, and a number of restaurants & cafes.

If you're both into food then Borough Market is an interesting place to look around. El Pastor tacos would then be my suggestion of where to eat.