Must do’s? If you enjoy theater, check out what’s happening at The Globe or in the West End. High-priced shows from the US are often a steal in London. There’s Harry Potter and the Cursed Child that fans might enjoy (3.5 hours, not a Steal, though).
The British Museum is chock-full of interesting artifacts. One of the greatest collections of Egyptian and Greek antiquities. Rosetta Stone. Guttenburg Bible. Admission is free, but require a reserved ticket.
Tower of London with the Crown Jewels. Guided tour. We had an excellent Beefeater lead our group. The Tower Bridge is right there.
Afternoon tea. Pubs with or without food or beer. Food from so many regions of the world. Borough Market in Bermondsey with food stalls (South side of the river across the Tower Bridge).
Walk around Buckingham Palace and the Queen Victoria statue, Green Park. View Westminster Palace (Houses of Parliament) and Big Ben.
Just walking around London is an adventure. Walk across the Milkenium Bridge. Take a HOHO bus tour to get an overview; double decker top deck if not raining. And ride the Tube - but mind the step.
"Why, Sir, you find no man, at all intellectual, who is willing to leave London. No, Sir, when a man is tired of London, he is tired of life; for there is in London all that life can afford." - Samuel Johnson, 1777. Only two days Pre-cruise?
(Members of my family have also stayed at Premier Inns and would do so again. And other hotels.)