You can do Paris on a Budget.
I have been a few times. The last time admittedly was with a friend who used to live there so we stopped with her friends, but the time before that I went with a collage friend and we managed to fly from Manchester to CDG, stay in a 3 star hotel (Bed and Breakfast) and it was less than 15 mins walk to the Champs Elysees and it only cost us £145 each.
Your problem is going to be the time of year your going ie Peak Holiday season.
If you don't have the option to book it and give her a card with a 'Surprise! I have booked us a weekend in Paris for [enter date booked, ideally at the end of September]!' and book the trip when the schools and Uni's are back in term, then I would recommend Krakow, Poland.
Beautiful city, the people are soooooooo friendly and its one of the oldest cities in Mainland Europe, with one of the oldest Universities too. The food is excellent and the prices of the drinks, food, transport and hotels (really cheap) more than makes up for the slightly more expensive flights. Most people speak English so if your Polish isn't that hot you will get by.
I believe its very similar to Prague (which isn't surprising given the location, they are pretty much next door neighbours) but without the stag do's and brothels
Edit: If you are going to look at Krakow, book the hotel direct with the hotel - dont book through a tour operator or hotels.com or whatever. We managed to get a great deal on a 3 star hotel for 3 nights bed and breakfast by booking direct. It only cost us (well me cause I got it for my bf's birthday) £58 total (approx conversion, pay on arrival)...madness!!!