The Mario tank has been one of my favorites for quite some time now, but I have to say that Lego train one is cool!! Having a toddler around, I have an abundance of those dang blocks around, I think I just found an excuse to build a new tank!! LOL
As for the DIY part, I have some PVC
joints in my 110g, but I didn't do anything fancy with them, just covered them up with rocks and a little sand. In my koi pond (just built it in the past few weeks!!) I'm in the process of making plant stands/fish hideouts with some milk crates, I cut some of the sides out of them and filed down any sharp edges to make them a tunnel to hang out in and set the plants on top.