I wonder if my understanding of the meanings implied in these sentences is right. Besides, I would like to know how common they are in every day English and whether they can be used interchangeably when you want to ask someone what they are going to do tonight - I'm asking so because of the approximation between "want" vs "plan" meanings.
What are you up for tonight? (this one bluntly means "What do you want to do tonight?")
be up for sth
› to want to do something: We're going clubbing tonight if you're up for it.
What are you up to tonight? (this one is about the 3rd definition below, the on related to "devising" and "scheming".
As far as or approaching a certain point. For example, The water was nearly up to the windowsill, or They allowed us up to two hours to finish the test, or This seed should yield up to 300 bushels per acre. [c. a.d. 950]
be up to. Be able to do or deal with, as in When I got home, she asked if I was up to a walk on the beach. This usage is often put negatively, that is, not be up to something, as in He's not up to a long drive. [Late 1700s]
Occupied with, engaged in, as in What have you been up to lately? This usage can mean "devising" or "scheming," as in We knew those two were up to something. It also appears in up to no good, meaning "occupied with or devising something harmful," as in I'm sure those kids are up to no good. [First half of 1800s]
Dependent on, as in The success of this project is up to us. [c. 1900] Also see the following idioms beginning with up to.
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.