I guess it depends on what excites her. All of the above are good suggestions, but there is something for everyone in London, so picking up on her personal tastes should help narrow it down
Nearby Regent’s park and hyde park are both good, often good for a weekend morning.
Pierre Marcolini chocolates are not cheap, but as the one-time enfant terrible of the Brussels chocolate scene, the flavours are wonderful (37 Marylebone High St, Marylebone, London W1U 4QF)
Scandi Kitchen is a good place to explore Scandinavian food, including a decent sweet (candy) pick and mix, but do warn here about the salty licorice. (61 Great Titchfield St, Fitzrovia, London W1W 7PP)
Loads of restaurants of all cuisines / countries in the area, so make it a ‘where in the world would you like to eat today’ experience.
I recall enjoying the Photographers Gallery (16-18 Ramillies St, Soho, London W1F 7LW) many years ago. It’s part shop part gallery if still the same as it was.
Covent Garden has a lively evening vibe on a summer’s evening. The tube station can get very busy, but worth it for the atmosphere IMO.
Lunchtime at Borough market has some good interesting ‘street food’ (the Ethiopean stall impressed on my last visit), but there are plenty of good lunchtime options.
Abbey Road (recording) studios within walking distance.
Sometimes though it’s the simple pleasures of famous sights, travelling on a red London bus, travelling on the tube (though I don’t know why - I find the tube a depressing place), seeing places on the Monopoly board, etc.
One big hint. Do buy a pocket a2z map, as it often shows how easy it is to walk rather than taking the tube. Many times people walk further to take a tube, than if they’d just stayed above ground and enjoyed the fresh(ish) air.
Hope this helps, but do come back if there are specific interests she has - they undoubtedly can be pandered to in London.