In all seriousness, while you are saying you can't drill into the ceiling, the reality is that you certainly can if you want to, but you have to repair the ceiling and paint it before you leave. Simply putting some drywall compound in the holes will not mask that there were holes there, and you will be responsible.
So, be responsible! Drill a speaker into the ceiling joist, mount it firmly. When you move out, put in ceiling compound, sand is smooth, then go through the $20 burden of painting the ceiling! It's not that hard to do and while it will take half a day or so to take care of, it's not really that big of a deal.
Seriously, paint is cheap:
Acquire the Speedwall 1-Gallon Flat Interior Ceiling Paint 1250-1220V 01, matte finish features a latex formula that resists splattering for your convenience from The Home Depot
www.homedepot.com