(Sorry, the glass is totally covered in crap and I have no intention of cleaning it very much.. Not only is it a "functional" tank but even moving a tiny chunk of rock in there kicks up huge clouds of crap in the water.. I would vacuum it but I value it as a pod breeding ground too much)

My 10 gallon fuge that sits atop the canopy. The righthand baffle is slightly angled to prevent any splashing from the water that spills over. That was supposed to be the "sump" part, but the bloody heater didn't fit (great planning there). Instead I throw bags of carbon in there. The intake from the tank (A Hagen powerhead 802 pushed water up from the tank) comes through a bulkhead drilled at the side of the tank near the bottom. A Prizm skimmer hangs on the right off the fuge, so that water level doesn't fluctuate TOO badly.
The left hand section is my fuge/surge. I have a bed of crushed coral to help act as a pod breeding ground, and various macros and seagrasses that were either on the rock or purchased. I also threw all the smaller bits of Tonga rock in there that I wasn't able to use for my aquascaping. I add little bits from time to time as well in case I ever need some to mount frags. There's also a large Rhodactus sp. mushroom in there - there were 2 that I brought home once from work (they both detached from the rock they were on) but one vanished.. The other attached and has tripled in size since then.
The ugly crap in the middle is my Borneman surge. When the water level hits the top, the black float ball will pull up the toilet flapper and dump the contents of the fuge into my display. Total surge volume is around 4 gallons (plus however much the powerhead pumps into the display during that flush) which happens every minute for around 30 seconds.
I have a few critters living up there. A cerith snail, an astrea snail, three periwinkle snails (they are a tidal species so actually do exceptionally well up there), and many bristleworms I see at night. It's fun to chase 'em with the flashlight.