This is coming with a big disclaimer: CrunchLabs, the company founded by former NASA engineer and YouTuber Mark Rober, says that its Hack Pack is for people over 14 years old. However, my 8-year-old put it together with minimal help in an afternoon, although it did take him much longer than the 60-minute estimated time for a teenager. ("Why are you so worried, Mama?" he admonished me when I showed skepticism. "The drawings show you exactly what to do.") Each box has a robot to assemble that illustrates a STEM principle, complete with a battery and an Arduino microcontroller. You can also unlock a video, but the drawings in the included manual were clear and big enough that we could put it together without the help of the internet.
Our box had the infrared turret to illustrate the principle of infrared communication. My 8-year-old was thrilled, hiding it and using the infrared remote to ambush his sister with a barrage of foam darts. You can also unlock a web-based coding console to hack the robot by locking out other users. It's expensive, but the price-per-payoff valuation is worth it—the STEM principle it illustrated was interesting, the build process was engaging without being frustrating, and the project itself was very fun.
★ Alternatives: I love Mark Rober, so this is obviously the best box, but there are a ton of different science boxes. Kiwi Crate has a dedicated science crate. For much younger kids, Little Passports has an Early Explorers box ($25/month) aimed at children as young as 3. Science Expeditions is aimed at children over the age of 8; the projects are a little bizarre, like extracting the DNA from a strawberry. Neither my child nor I knew what we were supposed to do with this. Finally, a lot of people like Steve Spangler's Science Club STEM kits ($35), which are huge and come with multiple projects. These are fairly simple, like a marshmallow masher that you could Google yourself and make with stuff you probably have at home. However, I thought it was worth it to have the lesson and objects bundled so I didn't have to drink and empty a plastic soda bottle myself.