Visiting from the United States for June 20th with my best friend for her birthday!

Hi guys! I will be visiting london on June 20th with two of my long time friends. One of my friends happens to turn 22 on that day as well. We want to do something tremendously special for her, but as its all of our first times in London we werent sure what to do. We could do all the touristy stuff, but at the same time we want to make it a truly special day. If you guys have any tips, advice, or recommendations on where to go or what to do, I would appreciate it alot! Thanks!