When exploring the first location, near the first force field, go to the right and look up. You will notice a beehive, which you should destroy using the hook. You will unlock the first character - Bleez.
To unlock the second character, go to the second location and approach the object shown in the picture (right near the precipice). Throw Robin's spear at the target and you will receive Green Lantern Warrior Character Token.
Talk to Bat-Mite in the second location (at the beginning of it) and learn what item he needs. Approach the object nearby and switch to Batman. Use Sensor Suit to find another object on the ground. Approach it and dig up a helmet, which you should give to Bat-Mite. As a reward for completing the task, you will receive Atrocitus Character Token.