I am looking for a game that would be suitable for teaching basic orbital mechanics concepts to primary school students (grades 6-8). Contrary to most of the questions here asking about spaceflight simulators and such, I am not looking for super realistic representations of orbital mechanics. I want to allow students to tinker around with basic central force motion and see the ways in which conic sections are altered by thrust, etc. Seeing/enacting an example of rendezvous (maybe in a CW frame?) would be neat too.
I think Kerbal Space Program is going to be a common answer, and it is suitable, but unfortunately the fact that it is commercial makes it tougher to use in an educational setting. Extra points if there is something browser based, but my hopes aren't high for that.
EDIT: Wow, blown away by the variety and helpfulness of responses! Really love that this site exists. I can see why the question had a close vote for opinion based, as there are multiple answers here which in the absence of others could easily be accepted. In the interest of closing this out I am going with the wisdom of the crowd and accepting the top answer. And while I know that it's not the same as that sweet, sweet reputation, I am super thankful for all of the time people put in to giving helpful responses. Honorable mention to @Paul for a great hands-on method and @Steve for a really cool browser-based answer.