My favourite general activity is an old-schoolctivity called 'Alibis' or sometimes called 'Guilty'.

http://esl.about.com/cs/teachingresources/a/bl_guilty.htm 

In a mixed level class, stronger students can be paired with weaker students, so that they have to 'coach' the weaker students a little.

Other than that,  for more general activities - I like to use the kind of 'mingle'activties that restric movement a bit more.   For example, a common game is to:

have ss write one 'quiz' question about themselves on a given topic (say, "your job"), but tell them not to tell anyone else.

ss stand in two lines facing each other

ss question the person they are facing for 2 minuteson the topic (note - they don't know the quiz questions of each other yet!).  Tell students to remember everything they can about the other person.

After 2 minutes, the first person from the left line goes round to the back and everyone in the left line moves up one person.  They start again.

After all the students have spoken, they go and sit down in teams.  Each team quickly compiles a quiz about thier team members from the questions they wrote earlier.  Then the teams swap papers and have 5 minutes to discuss the answers.  When they finish, it's handed back, marked, and the winner is anounced.

In mixed level classes, I find the 'two lines facing each other' format gives weaker students nowhere to hide and forces them to speak with stronger students.    Also, in the final feedback, it is possible thast weaker students know the answers to questions the stronger students don't, so again they have to speak.    The freedom of the questions means that students can use whatever language resources they have to ask and answer.