Difference between What are you up for tonight vs What are you up to tonight?
Solution 1:
Colloquially, if A asks B, what are you up for tonight?, A would be inquiring what B would like to do tonight. There is an implication contained that A is interested in making or proposing an arrangement.
If A asks B, what are you up to tonight A would, ostensibly at least, be inquiring as to what B's plans were for the evening. Equally A may be about to suggest something but that much is not implied in the question, as it would be in the previous example.