Maniac Mansion is a point-and-click adventure game, originally released for the Commodore 64 and the Apple II in 1987. It was later released for MS-DOS in 1988, the Amiga and Atari ST in 1989, and the NES in 1990.
You can play the MS-DOS version at Archive.org for free. Click the game screen, then if your browser pops up a message asking you to disable the mouse (or something similar), allow it. You can right-click to cancel cut scenes and messages. There is no way to save the game in the Archive.org player. Read the MS-DOS Emulation FAQs at Archive.org for more information.
The following walkthroughs were written for the NES version, so there might be some differences in gameplay if you play other versions.