Sites of Interest
Location: Trick Track Trek
Items: Red Balloon, Green Balloon, Blue Balloon
Walkthrough: Pounder the last Manky Kong near the exit, and jump down below from his location. There's a hidden platform below that should catch you. There, you'll also find the barrel that will send you to the bonus room.
This is not different from the previous bonus rooms in DKC (ie. Bonus Room 39). You can claim one of the items in the barrels by matching all three similar items. Go for the Blue Balloon for a 3 lives up.