Thanks everyone, much appreciated.
Austin- this is our first trip to Europe, but both of us have been out of the country a number of times (mostly to Mexico and Central America).
I'm extremely lucky that this girl is seriously the second coming of Ferdinand Magellan. She can navigate a map like nothing I've ever seen, so a trip to the countryside with little to no plan outside of Stonehenge is 100% in the cards.
We've gotten a lot of people who recommend the Eye, and of course the museums and libraries will be the first day or 2, as she's also a fucking nerd who's into that stuff.
Like I said, we're considering taking a train to Barcelona, Germany, or now Paris, but since it's only a few weeks out it looks like tickets are expensive as hell. We are holding a day or so to book our AirBnB to see if we can get there a cheaper way, but they're like $250 a person right now, so that's out.