Word for either indoors or outdoors
Solution 1:
I would use environment:
[Merriam-Webster]
: the circumstances, objects, or conditions by which one is surrounded
Context would make it clear what the possible choices are.
While starting with indoor and outdoor, you could add mixed, as you say.
But it's a lot more flexible than just that. The environment for the activity could also be something like underwater, if it involved swimming.