There are loads of online learning sites available for adults, but the best list I’ve found by far is here:
It lists free MOOCs (Massive Open Online Courses) from great universities, with most offering certificates or statements of completion (sometimes at a small fee).
There are a wide variety of learning categories, such as Business, Computer Science, Economics, Environment and Mathematics. They are listed by upcoming start date, but if you scroll down to the end of the page, there are courses which may be started at any time.