To continue with the inner tube idea, dunkin donuts could provide a local pool with the inflatable donuts and then sell their product at the concession stand. People would see the inflatable version with a logo and perhaps something showing they are now available at the consession counter. They would then be tempted to indulge in the donuts rather than other snacks available at the counter. Dunkin Donuts could also offer to sell their iced coffees and other products at the concession stand. Since many donuts stores have sales trucks for outdoor events, selling donuts at a local swimming hole, tubing river, or other outdoor event should be simple. The ease of access to the product would certainly make specialty advertising more effective.
The company could also hold a contest for "dunks" (either basketball or golfing with a munchkin (the donut holes the company sells). The contestants could post their videos on a social media site for voting. The winner could get a commercial spot.
To continue to play with the floating donut idea, you offer a giveaway of inflatable Dunkin Donuts and market the give-away as a summer special. It is swimming season in the northern hemisphere now and it's ripe time for swimming and water related advertising.
I like the inflatable "donut" idea. To work on the other word, for Dunkin you might look at a basketball angle. If there was a celebrity endorsement a mini figure of a basketball player "dunkin" a ball. Or a small basketball or "dorm room" wall goal with the Dunkin Donuts logo on it.
I suppose the obvious thing here would be to make something on a donut theme. You could do something like giving out key chains with a donut as the holder for the chain. Or you could give out inner tube like things (for kids to float on in the water) that were decorated like a donut and had your logo on them.