1.) Radioactive human remains found in Harrapa and Mohenjo-Daro.

Any link to a non-kook website that this is actually true?

2.) Cocaine and Tobacco residues found in Egyptian Mummies.

There are Old World plants that have nicotine or cocaine in them, although in lower concentrations then the usual New World sources.
Considering that the dog is the only domesticated Old World species found in pre-Columbian America, and that the Amerinds never developed bronze working, the notion of ancient trade routes between Egypt and America should be approached with considerable scepticism.

3.) Antikythera Mechanism.

Is a mechanical calculator. Ancient writings mention similar devices.
This is an example of lost technology, at least in the West.
In Byzantium and the Islamic world during the Middle ages less complex mechanical devices were build.

4.) Baghdad Battery.

What's so unexplainable about batteries?
They are extremely simple batteries. If that is what they were. There are other theories.

5.) Strata from the Earth depicting fossils of both dinosaur and human footprints.

Only according to some creationists.

6.) Ancient plumbing and piping in China.

Chinese scientists have established that the Baigong Pipes are fossilised tree casts.

7.) Crystal Skulls.

All known crystal skulls that have been properly investigated are of relatively modern manufacture.
No crystal skull has a provenance showing it came from a pre-Columbian source.

8.) Giant Balls of Costa Rica.

Are giant man-made balls carved from stone.
There are lots of artifacts made by cultures now long gone about which one can only speculate about there purpose.
Their 'incredible precision' is vastly exaggerated. It is a kook myth.

9.) China's First Emperor's Pyramid.

Why is this even on the list?

10.) Buildings with interlocking segments Puma Punku.

B.S. The builders of Pumapunku were master stone-cutters and great engineers.
'Could not recreate' is just another kook myth.
Kooks have a tendency to sheepishly quote each other rather then to check the veracity of such claims.

11.) The Voynich Manuscript.

Plenty of hypotheses around.
One of them being that it is deliberate gibberish.
Anyhow, what exactly is 'unexplainable' about a code that hasn't been broken yet?
Isn't that what codes are for?

12.) Small Ancient Flying Machines.

From the very website: Explanations have been offered from children’s toys, someone’s imagination, a flying fish, or actual ancient flying machines either made by the local population or seen in the sky by them.

Any of these might be true. Or not.
Some are more probable then others, though.
The simple fact is that without more data one can't tell.

This is normal.
Filling in the lacunae in ones knowledge with fantasies is useless.