Things to do in London for first timer?
The Harry Potter Studio tour is fantastic! Not sure if street art is of interest but the Shoreditch Street Art tour is interesting. I have reviews of both on philatravelgirl blog.
At night there are options with walks.com for a pub tour, Winter Wonderland at Hyde Park if you like carnivals or live music at Scala. Look at Time Out app for events like multiple pop up food, music, fun during your visit