Start a game in Hard mode (or Nightmare if you've unlocked it) and select a bonus mission. When the mission begins, change it to Easy. The clock is longer on harder modes, but when you change the difficulty, the longer level time stays and the enemies are easy. When you complete the bonus level, the time bonus will be very high.
Go to the following locations and perform the corresponding tasks to unlock a Bonus Mission Map:
HOW TO UNLOCK
After launching from the caravan, you'll land in Paleo Pass right next to a Swap Zone. Use a character with the Climb ability to complete the challenge and earn this map.
After you battle the Tech Gear Golem in Fantasm Village, you'll approach a Swap Zone. Use a character with the Rocket Ability to complete the course and earn this Bonus Mission Map.
Iron Jaw Gulch
Activate and cross the bridge in 10,000 Gallon Hat, then enter the large battle near a Life Elemental Gate. Take out the enemies and use a Life Skylander to open the gate. Bounce on every platform inside to open the gate and claim your prize.
After you escape from the second Sheepshooter explosion, head straight forward to reach a Swap Zone. Use a character with the Climb ability to complete the course and receive this.
While going through The Secret Way you'll find a dual Air/Water Elemental Gate. Use a Swap Force character with these attributes (or two appropriate Skylanders, if you're playing co-op) to open the way. Complete the course inside to earn this.
Toward the end of the level you'll see this Swap Zone to the right. Use a character with the Climb ability to activate it, then complete the challenge to receive a Bonus Mission Map.
Enter Orangutan Tower and look to your left to spot a Swap Zone. Use a character with the Climb ability to activate it, then complete the challenge to receive a Bonus Mission Map.
You'll be near an Air Elemental Gate after beating the Boom Bosses in Perilous Plateau. Use an Air Skylander to open the way, then clear the course inside to earn this reward.
Complete Mesmerelda's Show to have this mission appear near Flynn's ship.
Upon reaching the point in the storyline where you get the chests in the chieftess' hut at Woodburrow that the person refills every day, open the chests and grab the coins. Once done, go to the "Level Select" screen and enter a level. As quickly as you can, open the menu and return to Woodburrow. When you get back into town the chests will already be refilled. Repeat this process for infinite coins. This results in about 440 total coins and takes 30 to 45 seconds to repeat.