Post-AP classes are similar to second-semester (or in some cases, like math, second-year) college courses! They’re basically what you take if you’ve finished the highest AP class before graduating. And yes, Multivariable Calc (sometimes Linear Algebra) is the first Post-AP Math that’s usually available.
I’m not sure about Post-AP History or English, but you may want to check your local college/university’s class list — see what they offer to those with AP credit; those are the classes that will most likely be offered.
Of course taking DE Classes (especially Post-AP level) will be useful! It can prove nicely as a back-up (in case you don’t want to pursue the humanities anymore, who knows?) or will allow you to be challenged adequately for the rest of high school! Plus, it looks great on your transcript, as a course that covers material harder than AP classes!