Jenny Bristol, Homeschooling Parent, Writer, and Editor
There are definitely plenty of challenging online courses for high school students, especially advanced students. With so many MOOCs these days, high school students who like to stretch their educational legs have thousands of classes at their disposal. But there are even plenty directed specifically to high school students.
My favorite source for this is EdX. Among almost a thousand classes are dozens (currently 61) classes aimed at high school students. Some of them are AP test preparation classes, but these could easily stand in for a regular class, and plenty of the high school classes they offer are not test prep oriented. They are taught and organized by colleges such as MIT, Rice, UC Berkeley, among others.
The classes offered definitely slant toward the math/science/computer science types, but they also offer Spanish, Government, Psychology, and others.
Also check out some more college-level classes for your daughter and see what might be a good fit. EdX has plenty, and there are many other MOOCs available online.