One day/night in London, first time visitor

Hello there!

Flying into London for a day prior to a trip to Greece this summer. We chose to stay a night since I have never been, and want to catch some of the highlights! We land at 9AM at Heathrow pending no delays from American Airlines (a bit of a stretch but I am hopeful), and we are staying at the Hyatt Regency Churchill. Planning to use public transit/walk most places. Would love any suggestions of what we should try to see without cramming too much in the day! Of note - coming in late July so we know it will be hot - coming from Dallas Texas so it's nothing we aren't used to when trying to enjoy the outdoors in the summer 😅

We like historical landmarks, good food/drinks, parks, etc. We leave at 11AM the following morning, so any quick bites on the way out in the area would be good as well!

Thank you in advance!