In this case, you are using a simile.
"Being in bed is as relaxing as one can get" would be a great simile to use.
Changing it to "as" is still called a simile but making it even better.
Mercutio is Romeo's best friend. Mercutio likes to have a good laugh, is optimistic, loyal, and a good friend. When Romeo is depressed because of his unrequited love for Rosaline, it is Mercutio that suggests they should all gate-crash the Capulet party.