what have you been up to? - it means I haven't seen you for a long time, so tell me what you have been doing? You can answer anything that kept you busy. E.g: "What have you been up to today?" - , "I was at work until 2:00pm, and then I went to the store to pick up some milk." Hope this helps :D

It can also be an accusation, for example if you were a teenager and got home very late, the parents might say this to you.

If someone says "what have you been up to?" Depending on when the last time you talked, you would basically tell them, "I'm in school, working full-time. etc..." what ever has been going on in your life that you would like to share with them.

what were the thing you've been busy with,.Its kinda like that.

What have you been doing recently? It's an informal way of asking how you have been and what have you been doing. :)