Glass tops are a good investment and also protect your dresser from other things like cup rings and spilt lotion/makeup/whatever. They don't always have to be expensive or perfectly sized. I picked up an antique mirror for $3 at a garage sale to cover a buffet, and used a $10 IKEA mirror on a coffee table.
My place also has small bedrooms and I have a luggage stand at the foot of each bed. I like that they're easy to put away if someone wants extra space, or when I'm cleaning.
You might also consider something that makes the dresser less of an option - like a vase of flowers, or a tray with carafe of water and glasses. You can kinda nudge people into using things the way you'd like by making their other options less attractive. Signs are a last resort when those tricks don't work. 😉