I know that Elon Musk has stated that SpaceX will try to launch the "silliest thing imaginable" on the maiden flight of the Falcon Heavy, but will it be a Dragon capsule on top or a special SpaceX satellite?
Elon Musk has stated that it is highly unlikely that the Falcon Heavy will actually make it to orbit on the maiden flight. This is due to the fact that it will be the first time that they can test out how the fuel pumps between the boosters work and also the separation and stage strengthening. Due to this, it is highly likely that the first Falcon Heavy launch will be with a fairing containing some form of test payload. A Dragon capsule would likely not be used due to pricing given that they would basically be throwing it away. One situation I can think of however where they would use a Dragon would be if it is an early Dragon 2 as it would allow them to practice launch escape with the capsule if something was to go wrong. In terms of what they will actually launch, it will likely be something like a test payload. Musk did say it would be something silly so you never know, his mind works in mysterious ways.
In Dec 2017, Musk revealed they'd be sending his old Tesla Roadster as the payload: