If someone is DTR, it means that they are excited to do basically anything. This is especially true of situations of craziness, ridiculousness, randomness, and spontaneity.
DTR can be a lifestyle. It can also be a mood.
friend - "Sure, man! I'm down to ride!"
me - "Later we can go climb some buildings and point laser pointers at drunk people!"
friend - "Totally DTR."
I´m down to ride