Dic: up to something

• I think they are up to something. 
• I am sure that Lily and Max are up to something evil. 
• I'm not quite up to the party. 
• Are you up to a game of volleyball? 
• This work's not up to the standard of the class. 
• Your last essay was not up to your best.

1. Fig.[of someone] plotting something.
2. Fig.[of someone] well enough or rested enough to do something.
3. to be as good as something; to be good enough for something.

.... http://idioms.thefreedictionary.com/up+to