Being really honest, if someone came to me and said "I have never been to Europe, and I have about $5000 for my wife and myself to see it"
I would not hesitate for a second to recommend a Mediterranean cruise that visits: Istanbul, Santorini, Ephesus, Athens, Rome, Florence, Monte Carlo, Barcelona, etc.
You can substitute Venice, or the U.K. etc, if you want. But I think a cruise like this, with lots of ports of call, is a fantastic way to see Europe.
Of course, it always helps to have a pre-cruise visit to at least one city to really see it up close.