I would use a sand gravel mix for the substrate in either white or black(my favorite). I would then use the space to build a variety of caves with texas holy rock or rocks of your choice interlaced with long viney plants and some driftwood. I would then fill the tank with cichlids from lake tanganyila and just watch the colonies grow. hope it helps and good luck.
LOL Toehead.You forgot the filter intake and outlet,heater,air stone and powerhead.
Seriously though.If I said what I would put in it,it would be what I like.Best to work out what fish you like first,then research on what you need to house them.Though if I had a spare 125 I'd opt for a pair of Jags.