One of my coworkers agreed to drink with me after work but we haven't settled on a time to meet up.
Why do you need to schedule it if it's just drinks after work?
I need to make sure it won't be on a day when my girlfriend needs me at home.
Making sure your leisure activities don't interfere with your responsibilities at home is a really good thing. It makes sure you don't make promises you can't keep due to timing or have to disappoint someone because you had to cancel.
Yeah, we just want to relax and have a few drinks, nothing major.
Even though you're only planning to hang out for a little while, it's good that you're making sure that you don't have anything important to do with your girlfriend first. That not only shows that you care about your girlfriend, but your friend from work as well.