If you know there is going to be a meeting in advance, splurge for pastries, or snacks, then put in a vacant conference room. When you get back and the meeting has started, say, "Oh, I thought everyone would be more comfortable in the conference room, so I went ahead and put pastries in there for everyone. Enjoy!" Once they're there, you might mention to the boss, in passing, how nice it is to see them enjoying their meeting in there and how much more they'll probably accomplish. Next time he passes by and they're having a conference in the cubicle, he just might ask them why they aren't making use of the conference room.