You didn't say when your girlfriend is visiting but I assume it's soon during the holidays? I think you have been given so far some solid options.
I definitely will second the notion of several:
*Ice skating at Triangle Park. $10 for 90 minutes including skate rental.
*Raven Run Sanctuary. Free (and beautiful).
*Southern Lights at the Ky Horse Park. $15 dollars per car if you go during the week.
*Holidays at Shaker's Village. $7 for the Christmas tour.
*Kentucky Theater. $5 for Christmas classic series shown weekends in December. Showing such classicsn as White Christmas and It's A Wonderful Life.
Some other suggestions:
Lexington Downtown Arts Center has 2 theater shows in December. The 12 Dates of Christmas and David Sedaris's The Santaland Diaries. $15 for one or $25 for both shows. They do have student discounts and you are supporting the local theater scene.
UK Arboretum. Free.
USA Dance Bluegrass chapter has a monthly ballroom dance night open to the public. The first hour or so is a lesson and the rest of the night is dancing. It is $5 with student ID.
I also suggest just walking around downtown Lexington around the New Court House and Phoenix Park area and just looking at the holiday lights. Very pretty down there.
Pazzo's is a great place for pizza and drinks. Joe Bologna's is also good for Italian and is located in an old church. Both places are fairly reasonable as long as you don't go overboard with the drinks. If you want to watch a movie while you eat and have some drinks, then check out Movie Tavern.
Nightlife, go to the Penguin Piano Bar. Always a fun night there and you can also check out the bars/clubs in Victorian Square next door. I also love to take people to Charlie Brown's. Nice place to relax on old couches surrounded by old books for a few drinks or a quick bite.
Although not holiday related, I always suggest to out of town people a day at Lexington Green. You can watch a second run movie in the early afternoon. They also have 3D movies which is rare for a discount theater. I think even for a 3D movie, its 4 dollars per person. Non 3D, $1.50 or $2. Afterwards you can have some frozen yogurt at Fro-Jos. Walk over to Whole Foods where there is usually a free wine tasting on the weekends. Spend some time at Joseph Beth Bookstore which can keep you busy for hours. They even have a nice little bistro in there that is fairly cheap (or at least cheaper than Tony Roma's or the Regatta also by the bookstore). And you can finish the night off with a show at Comedy Off Broadway.
Lexington has a lot to offer if you look for it. And being so close to Louisville and Cincinnati which offers up ever more things, I am sure you can show her a good time in the area without breaking the bank.