Decontamination Chamber: Fortunately, these days, it just involves a light mist of nanites checking you out (while clothed, so no fanservice, alas), but it’s routine protocol for (physically) entering a new habitat or, indeed, planet, given the risks of disease and, worse, ecological contamination.
It probably tells you everything you need to know about the Empire that you can bring all the weapons you like along on interstellar trips, but gods help you should you smuggle a single unauthorized microbe…
Alien Hair: Sometimes. Also, sometimes, fur, tentacles, symbiotic fungi, cilia, etc.
Alien Geometries: Something seen in automation-dominated infrastructure areas, where buildings and systems which are – for the most part – only ever used by AI tend to be made in complexes of simple geometric shapes, lines, and curves, without any of the softening or decoration that most sophont eyes find comfortable.
Alien Blood: Played straight, in many varieties. For example, eldrae blood is close to indigo in color, due to the not-hemoglobin it uses (borrowed from Elieran bluelife, which is indeed mostly blue; and it would lead to noticeably different skin tones if they weren’t all so damn pale anyway). Kaeth blood is silvery-white, and notably conductive. Myneni “blood”, really crystalplasm, is whitish and feels sandy. Esseli blood is red-orange, and mirilasté blood is a fluorescent magenta.
The Aesthetics of Technology: Oh, the Imperials play this one completely straight. After all, the Imperial motto is Order, Progress, Liberty, and it would never do to make all those neophilic people think that they weren’t getting their Progress, capital-p included. So not only should it be all future-y, but it should look that way, too.
Which is why the stereotypical piece of technology looks like an Art Deco iPad rendered in organic shapes, out of burnished metal and crystal and gleaming stone and oiled wood, with some probably unnecessary glowy bits added around the side, and generous use of trigraphic (“hologram”) interfaces displaying all the information you need and quite a bit of information you don’t need but might quite like to have anyway. Larger industrial machinery takes this and adds a few tons of brass piping on top. Just to really take the futurity up to eleven, most of it is happy to respond to thought commands without you actually having to use a physical interface at all, although – especially on the large industrial machinery – it’s present anyway just in case you need it, and because it wouldn’t look machine-y enough without it.
And it’s all polished until it gleams, because it’s supposed to look beautiful as well as impossibly high-tech. These chaps know their market.
Action Bomb: Spider mines. Also AKVs (Autonomous Kill Vehicles) who can use themselves as a kinetic-kill weapon when they run out of ammunition. Of course, they back themselves up first, so it’s not like they’re blowing themselves up permanently.
Abusing the Kardashev Scale For Fun and Profit: Working their way up to this right now, the Empire has a couple of Dyson spheres, one under construction, the other completely dedicated to energy generation, plus other energy-generation facilities scattered here and there and about the place. This makes them, I believe, a solid Type II, although given the constraints of light speed, it’s going to take them a while to work up to III.