One of the things I hope comes from Contact with (intelligent) nonhumans, when or if it happens, is the ability to create megalithic structures like the ancients (human and/or not) did with the pyramids etc. We will or would finally know just exactly how the huge stones are carved (with laser-like precision), moved and placed. What a marvel it will or would be!
I would think we wouldn't have to limit ourselves to rock quarried on Earth. There's a whole solar system with plenty of moons with enough solid rock. Picture a pyramid made from rock found only on (the Jovian moon) Ganymede or wherever. That would ensure a legacy of its plainly non-Earthly origin, thereby eliminating the possibility of any future controversy of the highly advanced, spacefaring technology of the builders.
I've come up with a novel use for a (new) pyramid, and it's a bit sarcastic, and probably won't really be necessary when the time comes (due to the inevitable revelation of the truth about/behind 9/11): a crash test of a Boeing 757. Picture it being flown (remotely-controlled, obviously), at an even higher speed than the one on 9/11, into the pyramid or whatever large megalithic structure - an edifice much more rock-solid than the Pentagon. The results, guaranteed, will/would be that there will be some remnants of the aircraft - something of (especially) the engines, wings, tail, fuselage, not to mention the inner contents such as seats. Whatever actually happened on 9/11, in the least, is completely outside of known physics (at more than one location).