I wouldn't personally do the Paris daytrip but I know people who have come to London from a long way away and thought that it was worth taking their chance!

The Thames walk is good fun in central London and fun in a different way outside of London, through the countryside. You can start at one railway station and walk to the next one (for instance, Oxford to Abingdon, which is the bit I'm most familiar with... some great pubs en route).

https://www.thames-path.org.uk

If she decides on Oxford, I can offer more detailed recommendations. Or, you know, if she fancies meeting for a cup of tea or something.