Round the sides
The left side panel of the 690 has two perforated areas, one over the place where the CPU on an ATX motherboard resides and the other where a PCIe graphics card would sit. A 120mm fan is pre-fitted to help keep the graphics card cool but it's attached to the steel panel and does cause some vibration when it's running.
The second perforated area lacks a fan but does have the same fixing holes as the lower area - so can hold an 80mm, 90mm, 120mm or 140mm fan.
Those options are welcome but Cooler Master hasn't blocked up any of the unused mounting holes and these do look a little messy. A few plastic caps would have made things look a whole lot neater and more professional.
On the right side panel, there's a single perforated area directly underneath where the CPU sits on an ATX motherboard. This can hold an 80mm fan but that would have to be low-profile (15mm) so as not to foul the motherboard.
It's another interesting idea but we'd expect an increase in noise - from the fan itself and from vibration of the side panel. We didn't fit a fan to see if that's true and so can't be sure how much benefit a fan would bring and whether or not this unusual option would end up being just a noisy gimmick.