I made an office block with a central support of glowstone (taking about 8 hours in the nether...), but I dug right down to bedrock, and made a dorm for workers, industrial rooms like with ovens and crafting tables, a reception at ground level, office cubicles, a hotel, and then a penthouse suite for me to live in with all the redstone wonders i could think of... and then last night on a mate's creative server (normally a survival guy) I made an airship with viewing deck, propeller, and a first class cabin. on the survival side of things, i usually build into the coolest place i can find (lately in a waterfall cave), and build with the natural formations guiding the construction, until i find any inspiration later to make something else, or until a mate asks for help.
the inspirations for these came from:
reading about skyscrapers on the msn homepage (hence giant tower)
a youtube video with balloons made of wool, and Archer episode Skytanic
just seeing what was there and building out of neccessity
the last was probably the easiest to get inspired by, but if you see something on TV, the net or in a book that interests you, use that as a basic template and it can lead to something really impressive... alternatively, do what i initially did: build a skull, go in through the mouth, and build a glass temple underground that you can only enter through the base of the skull, following a pathway of tests of faith, and then have all the outside of the temple swimming in lava... always good for making you go insane building it, but loving the outcome