There are a number courses that have taken my fancy from the Future Learn website.
All the courses here are free courses and they have some fascinating quality short courses you can do online.
The first one I did was the basics of science (access here) which I actually did alongside the twins, we where also doing an online game at the time how to tame your dragon (access here) which does give some real science questions and answers and is pretty educational, so this was also great to do at the time *UPDATE, I have actually come across a great resource since writing this post, for how to train your dragon which is from the Chatterbooks Reading agency which you can download here.
The courses are for adults but I have found that the twins also enjoy doing them to a point, we just did the moon course (access here) where the twins where fascinated.
The great thing about these courses is that they do repeat themselves so you can go back and do them again if you wish to.
The actual catalogue of courses they offer are amazing, there is so much to choose from, the only downfall is there is not enough time in a day to do all the courses you want to. And they are all FREE, yes FREE.... how amazing is that!.... the quality of the content is excellent, and they are also very interactive.
I really cannot recommend them highly enough...