Generating maximum wifi activity through 1 computer
Simply sending packets as fast as possible to a random destination (that is not localhost) should work.
You'll need to use udp (otherwise you need a connection acknowledge before you can send data).
cat /dev/urandom | pv | nc -u 1.1.1.1 9123
pv is optional (but nice).
You can also use /dev/zero, but there may be a risk of link-level compression.
Of course, make sure the router is not actually connected to the internet (you don't want to flood a server somewhere!), and that your computer has the router as the default route.