Story-based and cannot be missed.
*** Spoiler - click to reveal ***
1. In the Future, on the 2nd floor, Put the roller skates on the mummy, push it downstairs
2. Wear the tentacle flag and enter your mummy into the human show
3. Put the nametag on the mummy
4. With Bernard, Activate the stereo in Green tentacle's room
5. Get the barf from the entrance room
6. Send the barf to the future
7. Get the barf, go to the attic and use the barf on Harold to disqualify him.