The Tate Britain is the older, smaller and quieter brother of the Tate Modern -
it hosts a wide range of both classical and modern pieces by famous artists such as David Hockney.
Honestly I prefer it to the Tate Modern. There’s a wide variety of art to see, you’re not battling huge crowds and they do a much better job of contextualising what you’re looking at. Many rooms are themed around a historical event or time period and link each artwork to it.
There’s a boat you can take from the pier outside to whisk you straight to the Tate Modern after you’ve visited
if you want your day to be entirely art themed but it is pretty infrequent.
My only disappointment was that the cafe isn’t great, I wouldn’t go for lunch. You can solve this though
by heading across Lambeth Bridge and having lunch at the Garden Museum.