You could tie in a boat trip with a visit to Greenwich. Use the Thames Clipper to get there from central London, you will see all the main sites from the river.
The Cutty Sark is always impressive, even if you don’t go in. The Royal Observatory is free I think, but they also have shows that require booking, some free and some cost. The Maritime museum had a similar refurb to the IWM, just seems a bit too vague to me, but you can buy joint tickets for the Greenwich attractions.
Have a look here if you are interested;
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Get a picnic together in Greenwich and eat in the park below the observatory.
London Buses no longer take cash, you will need an Oyster or Contactless Bankcard for individual tickets. Travelcards are only available after 9.30(?) unless included with a train ticket but get cheaper travel on the Thames Clipper service too.