You can be up for {something}
and that means you're amenable.
Are you up for a little tennis?
But if you're feeling tired, or achy, or a little under the weather, you might reply:
Sorry, I'm not quite up to it.
That means you cannot summon the energy or will to do it, or that there is something hindering or preventing you.
So, if you are up to it, you can summon the energy or stamina or will or desire. You're willing and capable.
We were thinking of taking a 15-mile hike up into the canyon tomorrow. How's you ankle? Has the sprain healed?
-- Sure, I'm up to it. I'd like to come along. The ankle's good.
plan isn't an activity, but planning is, although it doesn't mean that you're willing and able to hang out with someone.
Your wedding is in six months. Don't you think you should start looking for a venue? Or do you want to get married on a beach somewhere?
--You're right, it's high time I found a place, but I'm just so stressed out with work. I'm not really up to planning the whole thing.