I read your other question (not showing up on the questions page, I think you need to change your settings on it so others can see it - you must have just set it to your local area) where you say your oldest is really into Minecraft and would play it for hours - and in fact, does.
You also call him an introvert. I'm a parent of introverts - so can relate.
I have been a stay at home mom at times. I can relate to this also.
So the thing is, some kids are more naturally comfortable being at home, and doing quiet things, on their own, or with each other (siblings) now and then - then being active, and out in the world all the time. It's still important (I think) for them to get out now and then and to connect - as you're saying, this concerns you too. You went on a visit with friends the other day (the Lego 'incident') so that's cool.
Some of my kids were not into sports. They just aren't that suited to it - but I would put a badminton net up, or we have a pool (even one of those inflatable ones, or if you're near a pool/ocean..) or even bicycles .. will get them out. It does not have to be a team sport. Throw a ball around. We fill up water balloons and the kids will have a fight on a hot afternoon.
Just break up the Minecraft.
As for being creative - Minecraft can be creative. I've enrolled one of mine in a programing course this summer. Very introverted. Other camps didn't appeal.
Invite kids over they get along with. Introverts can be slow this way - I sometimes have to really take the lead on this. One of my kids basically had to be 'told' to have friends over. Sometimes you have to gently push.
We tried a drama camp - my kid didn't really do acting, more the narrator part of it but enjoyed being a part of the social scene, but was exhausted at the end of the day. If your kids would do a sleep away camp that's fun - even for a 2 night one. If not, there are day camps that are geared towards every kind of interest these days. There are Minecraft ones here. They break them up so there's some kind of physical activity and other things going on, with some social activity too.
I wouldn't worry too much about it - introverts seem like they're not getting enough .. but so long as they are happy, they're ok. If they are bugging you (bored, needing things to do) then supply them with board games, a jar of ideas (you can get them on pinterest), where you have two kids, some things for outside to do (some games), and say they have to leave you alone for an hour (or half hour) to start and they have to entertain themselves.
Give them a simple recipe to make (slime..) something. Good luck :)