In case you decide to go with a fan, you can pick up a small 40mm or 50mm 12v fan and run it at 5v off the GPIO header. I did this on my odroid C2, using a 50mm (I think) fan I stole from a dead and very old Buffalo NAS device and just screwed on the top of their standard case.
Because it is running at less voltage than it is designed for, it spins a lot slower and is therefore extremely quiet (read: darn near silent to my ears) yet moves enough air to keep temps under control. My odroid was running 150-160F (65-70C) without the fan, and with it temps are down in the 100F (38C) range all the time.
On my Rpi3, I went with the C4Labs Zebra Virtue with their 5v fan... which is a little too loud, but the case is really nice otherwise. It does keep temps down in the 105F (40C) range with the aluminum heat sinks they provide.
