Under the guildhall art gallery in the city of london, there are the actual remains of a roman amphitheater from around 70 ad. It's usually pretty quiet because most people don't even know it exists.

The gallery itself is free and has a decent collection, but the amphitheater ruins in the basement are the real hidden gem. You can see the original arena outline and imagine gladiator fights happening right there 2000 years ago.

Perfect when you need a break from tourist crowds. Opens 10am daily, bank station is closest. Takes about 30 minutes to see everything properly.