You can find this Big Challenge in the Much-2-Much region of Peak III. Much like the Pop The Kitty Big Challenge on Crow's Nest, you must pop three balloons that are floating above. To do this, move forward with boost and do not jump at the ramp. You will hit the balloon. Hug the right ramp, but turn left while boosting. Jump at the ramp, and you will hit the second balloon on the other side. Boost hard to reach the next platform, and boost harder on the left platform to burst the last balloon. As always, do this mission over again for a harder difficulty.
You can find the final Big Challenge in the game at Much-2-Much. The objective to this mission is to simply go through the course with over 23 seconds of air time. This is not too insanely difficult, as you will just need to hold all of your trick buttons to gain more time in the air. If you're still short, try jumping in the air on the regular course to complete the mission.