You should post that as a topic with its own thread as you will probably get better answers that way. Personally, I've never had to make up excuses for this except to get away from co-workers on business trips and for that I usually say I have friends in town that I am going to see. I am sure a similar excuse of going to hang out with a buddy from work or going with people to some event (like a baseball game) would be good if the excuse is for a significant other.

If it is for work, then having to take your car for some repair, doing something for a family member in need or even a family pet are ideas but that really is dependent on your boss.