The thing you are "up to" is what you have been doing. Example: "So what have you been up to lately?"

"Up to" can also mean doing something wrong. Example: "I think those kids are up to something!"

"Up to" can also be a measure of how good something is. Example: "This is going to be a tough game. Do yo think you are up to it?" Reply: "Absolutely."

If you are "up to it" then you are able to do it. Example: "We can play another game, if you are up to it." Reply: "Let's do it."