Try to "do it all", as it's really great - be sure to do the museum, and be prepared to spend some hours there. If you go on their website, it lists the events scheduled for that time period (polo, horse shows, etc).
I live about 15 minutes from the Horse Park, e-mail me privately if you want hotel info, etc (there are some good hotels in Georgetown, if you don't want to stay in Lexington).
And if you can manage it, contact some of the local horse farms and attend some tours, especially if there's a stallion or two you'd love to see. Claiborne, Three Chimneys, etc. The "tour bus outfits" are good, you will get your moneys worth. And Keeneland, you have to go see Keeneland!
Louisville is an easy hour away, if you would like to see Churchill Downs and the Derby museum one afternoon.
I moved to the bluegrass area about 11 years ago from the Chicago area, and just love it.