Where in the UK are you planning on visiting, London? Or do you want to go outside of London as well?
If you are looking for the scenery and water etc then in England the Lake District is nice -
As for the scenery though, you can't beat Scotland! I know, I know, I am biased I hear you say but I don't think many would disagree that Scotland has some of the best scenery and outdoor stuff in the world.
Warning, spring can be rather chilly here in Scotland though! And a good chance of snow still on the ground and definitely on the mountains then.
London is def teh place for museums though but remember that the UK is a tiny island compared to the size of the USA and travel throughout the UK is easy. Eg- I live in the north east of Scotland and once drove to London in just 6 hours so that gives you an idea of the size of the UK. A flight from London up to here takes less than 1 hour so I guess you would be able to do lots of different things if you and Aimee are prepared to travel a bit. Lainy's idea of a tour around sounds good to me.
Let me know if I can help at all and I'll do what I can.
PS - You will def meet some interesting people here and if you come to Scotland you will certainly here some very interesting accents!!