What is the opposite of "busy schedule"? I read the forum at this link, and it says "loose schedule" is not a good choice. What is the common adjective for this?
Depends a bit on what you mean by "opposite", but the best choice is probably "open". "My schedule is open" means that, while you may have some things scheduled, they are few and far between, so it should be easy for you to fit something else in there.
"My schedule is empty" goes even further than "open": it means you have nothing planned.
"My schedule is loose" would be a way to say that your schedule is flexible (though personally, I would stick to the latter word in this context). In other words, you have things planned — perhaps lots of things — but you can move them around if needed. It's perfectly possible to be busy and flexible at the same time.
"I have an easy schedule for tomorrow" means that the things you have planned are all simple tasks that you will be able to complete without much effort. There is also some implication that at least parts of your schedule are open, because part of being easy is not having any time pressure. (Peeling a potato is a simple task if you have 10 minutes to do it. If you only have 30 seconds, suddenly it's a stressful & difficult activity.)
If what you're looking for is a way to say the opposite of "I'm busy", i.e. without the word "schedule" in there, then your best bet is to simply negate the statement: "I'm not busy".
I've also heard people aiming for a relaxed schedule, meaning there's lots of downtime between individual appointments, allowing for meetings to be extended or picking up something to eat in between.
However, the term could also be understood to refer to a loose schedule, eg. that coming 15-30 minutes late would be completely okay.