As a single female of the same age, I frequently look for the same thing when traveling. London is my favorite, but you've been there. I would definitely suggest Paris. It's so large and lively and diverse. There really is something there for everyone. I've been at least 5 times now and i find new things every time. Perhaps Italy and take trains from one city to another, I prefer Florence, personally, but other are certainly worth seeing.

I always travel by train. It's easy and you get to see the country from the window.