There are plenty of freeware traffic generators available. If you want to do it for free, place a very large file on a server and download it 5 or 6 times simultaneously from the same PC or laptop.
Prior to doing this, open 12 command prompts, size and arrange them all and copy and paste the command ping (local server IP) -t into each. Once the download starts hit enter on each prompt one at a time.
That should give the network something to think about. Watch a video on BBC iPlayer, YouTube, or Netflix over the wireless at the same time just to really kill it.