The first item you unlocked was a shirt simply for having played Trials HD.
The second, potential unlockable item from Trials HD is another shirt for having unlocked the Marathon achievement in Trials HD. The Marathon achievement is probably one of the harder achievements around: "The player has completed the Ultimate Endurance tournament without any faults."
The third unlockable item, and I believe the last, is the Micro Donkey 60cc bike. That one is unlocked if you got the Unyielding achievement in Trials HD, which is rewarded for "The player has completed Groundhog Returns faultlessly without changing the rider's posture."
The shirts and bike can be found under the Garage. You can also read the descriptions for how to unlock them there. You can also view all the rewards from in game: hit Start, select Help & Options, then Select Trials HD Rewards, and you'll see the screen below: