On the First level of the town world, step on a switch in a barrel on the top of a small hill. The switch will open a door to the upper ledge, near the end of the level. Stand on the trap door for a few seconds. This will still allow you to complete the level, as well as go to the psychedelic stun chamber. In the chamber you must collect 50 of the green coins to play as the Tigress.
To play As sumner do the same thing with the trap door in the Trenches level of the Battle Field World. Collect 50 of the Yellow Coins to play as Sumner the Wizard.