There used to be a very useful applet for showing wave superposition
The alternative is to roll your own simulation from scratch, using a spreadsheet.
1) This is most likely easier than trying to find where the aforementioned
applet went.
2) This is IMHO more educational. Students are less intimidated by spreadsheets
than by general-purpose programming languages.
In particular, it can be a goal of the lesson (in addition to the obvious
wave-physics goals) to get students to see how such things are done.
It would be too much to expect naive students to create such a thing on
their own from scratch. Instead, give them a working version and ask
them to modify it slightly. This forces them to reverse-engineer it, to
see how it works.
-- Example: If you give them sin(wt) in column A, get them to put cos(wt) in
column B, and the sum of the squares in column C.
-- Example: If you given them sliders for frequency of one wave and frequency
of the other wave, get them to add a slider for relative phase.