N.. I don't know if this will be helpful at all, but fwiw - my EG ds is really into science, math and stem. He wasn't into online learning at all when he was 6 - wouldn't have mattered if it was dreambox-ish or cty-ish or whatever. What really worked well for him during the summer were week long camps, either through our science museum or outdoors camps where he was outside learning about things associated with nature. He also loved watching Nova and Nature (PBS) starting when he was really little (2 and 3 years old), but he still enjoyed them at 6. For brief entertainment he also enjoyed Mythbusters
The other thing I did with him was just spend a lot of time outside hiking, climbing on rocks etc. I realize it's not structured learning, but it was really enjoyable to him at that time in his life just to have his mind free to think about whatever, and being outdoors hiking etc all inspired creative thinking for him - he liked to dream up inventions etc. DH also used to let him tinker around in the garage a lot with him, and he enjoyed taking apart old things that didn't work anymore - we were able to find old radios etc, things like that, for just a few dollars at the second-hand store. I've never heard of the popular mechanics series you mentioned, but I'm guessing my ds would have loved it! I
Science magazines were a big hit with him too - we read to him from Scientific American etc, plus let him go to the used book store and pick out non-fiction books for us to read to him. He could read really well on his own by the time he was an older 6, but he still liked choosing books with us and having us read to him even though he could read. He would also hoard books on his bed - almost all non-fiction, and he never read them front-to-back but would browse wherever he felt like it at the moment. My only warning is beward of anatomy books - ds asked for one so we got him one, not having been fully prepared for a certain aha! moment that happened when he was studying a certain page...
Sorry I digressed! Those early years were just so much fun
We're clearly not a family that was too focused on acceleration, more so on just letting our kids explore their interests. And my dh and I are both scientists too, so we most likely would have been doing those things with our kids anyway because that's who we are.
Art was another thing that our ds really loved and that was fun for him - so don't discount other things outside of science either.
Re CTY specifically, our ds has taken upper level courses but didn't take any of the elementary courses. I'd probably *not* sign up for CTY at this point both because, as Val mentioned, the courses vary in quality, and they are extremely expensive - you can most likely send your ds to several summer camps for the price of one CTY course. We also found with our ds that sometimes what seems like the most dull and boring course through CTY might be really cool for him, and another that seems like it should be extremely engaging isn't - life's kind of like that, and I'm ok with all that, but CTY is a bit pricey for testing the waters in a course (to be fair, they will let you transfer your $ to a different course). CTY was helpful for ds at the point that he took them because his intent was to use them both for the learning experience and for subject acceleration purposes - but at 6, we really weren't looking at subject acceleration or needing any documented proof of what ds had learned, because of the way science courses were taught in our school district, there wasn't much of anything to gain in terms of specific acceleration until middle school, and we weren't looking for a full-grade acceleration.
Best wishes,
polarbear