I'm Going to London!

I've booked my flight to London over the break. And I'm almost certain I'm going to Paris.
These cities aren't on the top of my 'to go' list (surprisingly), but they're up there. It'll be a good change of pace from small little Reykjavik.

Now, the headaches start: what to do while I'm there and how to do it so that I maximize my time?

I'm really angry that I'll have to pay for all the accommodation on my own, which is the most costly expense in these two cities. I was counting on a (distant) relative and a (distant) friend to board me, but both of them are travelling. Boo.
I could've gone somewhere cheaper but I really don't know where.
I had an opportunity to pester some people and go to the north of France or Prague (for Christmas? how romantic) but it'd be another headache figuring out how to get there. Those can be for later.

Anyway, I have to tell myself: no regrets. What's decided is decided. Besides, I can list a few people who'd kill to go to London and Paris right now...