Most of the time what are you up to is just polite chit chat. Most Brits are horrified when anyone actually answers the question, especially if they go into detail.
Rules of engagement.
How are you?
Answer, fine thanks, you?
Good thanks, what are you up to?
Answer, not a lot, you?
Yes not much, just off to Tesco.
Answer, oh don't envy you, see you later.
Yes, see you later.
Rules of non-acceptable conversation
How are you?
Answer, oh, so, so. Better now my piles have improved, like sitting on pins it was. I'm at the hospital next week with me leg, and then we're at the clinic on Wednesday with Bert's knees....
oh, I'm sorry about^ that, what are you up to today?
Well we're off to X shop for biscuits then, we've got to go to Y place to get dog tablets, then my Lucy want z and she says you have to get the ones from Taylor's because Browns are no good...^
Half an hour later. Must dash have a good day.