Layover in London

Whats up OG-

I am going on a little vacation at the end of the week and am flying into London on my way to Madrid.

Getting to London around 4pm local time and leaving the next morning. Any suggestions on what I can or should try to do in limited time for that one night? Going with lady friend so has to be something we can both enjoy.

Thanks in advance for the tips!!

also any suggestions of Madrid or Amsterdam are welcome as it will be my first time there as well!