Ohio State has a free mooculus (calculus) course - https://mooculus.osu.edu/; accessible through coursera as well.

I can't say how it compares to Alcumus though or on the fun element, but I know kids who loved it.

EdX has a pre-calculus course. I think it's through U Delft. I think edX has some pre-calculus courses and others for free as an alternative to Khan and IXL. Coursera probably has other free math courses as well.