What to do for a great day out in London and surrounding areas?

I was born in Essex, and have spent some time of my life in London, however I now live in spain.

In about a month or so I will be flying to the UK to meet a friend for a fun day out, then I will return to Spain.

What do I spend the day doing?

My friend is female, I am male, and we are both in out late 20’s. It’s not a date, however some date-like activities would be ok, e.g. a movie.

My friend does not really care for theatre or fancy dinners, but is up for more or less anything. Having said that, she is very conservative and proper, a nightclub or casino are probably no good either.

I know some things we could do, and I know how to google “day out in London”, but really, I have lost touch, it has been years since I was there, and I have no idea what is hot at the moment.

My friend is also not from london, however happens to be based there for the time being.