Advice on day trips from London

Much depends on individual preferences.

>Is there enough to do in Windsor?

For a day visit? Most definitely yes, if you visit the castle, walk in Windsor Great Park and/or like (mostly expensive and/or visitor-aimed) shopping. Otherwise, no.

>Any thoughts on Cambridge for a day trip? Oxford vs Cambridge?

The two places are much of a muchness in that most of their attraction lies in their ancient colleges (some of which you can visit though only at set times) & riverside walks. Both have ancient churches, lots of people on bikes, a castle (Cambridge's is just the motte...mound), plenty of ancient pubs, loads of cafes/restaurants/shops and several museums.

Both have lots of visitors but Cambridge tends to have slightly fewer than Oxford because a) Oxford is a shorter daytrip from London and b) there are lots of organised daytrips from London which include a visit to Oxford.

The fact that Cambridge has an official tourist information website and Oxford doesn't tells you all you need to know about visitor numbers!

https://www.visitcambridge.org/

Both are worth visiting. The Pitt-Rivers and the Ashmolean museums in Oxford are truly superb but I prefer Cambridge's colleges (especially Kings college Chapel) and less visitor-crowded atmosphere.

Other ideas:

St Albans Very easy by train, magnificent and extremely ancient cathedral, Roman remains, lots of cafes/restaurants/pubs, plenty of shops & a decent market: https://www.enjoystalbans.com/

Winchester Ancient capital of England, lots of ancient buildings, superb Medieval cathedral, lots of pubs/cafes/restaurants/shops, pleasant riverside park: https://www.visitwinchester.co.uk/

Salisbury A wonderful Medieval cathedral, ancient buildings, lots of cafes/shops/restaurants, pleasant walks: https://www.visitwiltshire.co.uk/salisbury

Stratford-upon-Avon Train takes a couple of hours but with an early start it's perfectly possible to visit Shakespeare's home town. Ancient buildings, a river, plenty of places to eat, drink & shop...and usually lots of other visitors too: https://www.visitstratforduponavon.co.uk/

York: A couple of hours by train from London so a daytrip is feasible. It's a simply wonderful city with intact Medieval city walls a superb ancient minster (cathedral), loads of ancient buildings, churches & pubs, river trips, Jorvik Viking Centre, lots of cafes/restaurants/independent shops: https://www.visityork.org/

Seaside: Brighton for pebbly beach, the magnificent Royal Pavilion, the visitor-focused 'Lanes' with antique shops etc, oodles of places to eat & drink and a vibrant nightlife. Southend Clacton, Margate, Bournemouth , Littlehampton for classic (occasionally rather tacky) English seaside resorts...

I'll keep thinking.......there are a myriad of possibilities!

[ Edit: Edited on 5 Aug 2022, 10:01 GMT by leics2 ]

More ideas:

Rochester: ancient cathedral, big castle, lots of links to Charles Dickens, plenty of shops, cafes & pubs.....

Rye Hugely attractive ancient town once by the sea but now some distance away, strong links to historical smuggling, many really very old buildings including The Mermaid Inn (1420) and the Ship Inn (1592), Cafes, restaurants, independent shops, cobbled lanes.....

Bletchley for Bletchley Park, the place where wartime codes were broken (including the Enigma codes) and Alan Turing worked. Fascinating museums set in their original buildings, lovely house & gardens, very good cafe... https://bletchleypark.org.uk/

Leicester (has very, very few visitors) for an excellent example of how hugely multi-cultural UK communities can thrive. Visit the cathedral (12th-century) to see the brand-new grave of King Richard lll (killed in battle in 1485, the start of the Tudor dynasty). His remains were only discovered in 2012, under a car park. Go to the King Richard Visitor Centre next to the cathedral for the story and to see the original grave. https://kriii.com/ Lots of independent shops in 'The Lanes', excellent market, huge shopping centre, loads of places to eat & drink, New Walk Museum/art gallery.....

Suggest visiting Winchester, then Salisbury. I spent three days in the latter in June and enjoyed it immensely. Went on a daytrip to Stonehenge from Salisbury.