Can you enhance, somehow, the mechanical resistance of a soap film, so that it can actually separate two mediums or even prevent someone from crossing a window covered by this soap film?

Then it would just me a matter of establishing the film when needed, or disperse it. It would need some trick to be dispersed easily.

Imagine that we could find a way to tap into a tree's conducting tissues, and sap sugars from it. Not all of them, leave enough to keep the plant alive and healthy. 

There are two options. We could refine this sugar, and sell it, or we could try to go another (giant) step of the way.

Attempt to get electrical energy from sugar directly, somehow. I don't know a way of doing this that not include cellular respiration, If that is not possible, move on.  

Devise a chamber that consumes the sugar, through artificial celullar respiration. Instead of producing ATP, you would gather the electrons generated in the process.

The motivation to tap into plants isn't to be mean to them, it is actually to find a sustainable way to conserve forests. Rather than to uproot forests and replace them with crops, you would actively work along with the trees. It would be in your best interest then to keep the trees healthy.

A point of doubt is that should this idea of messing with plants metabolism hold, it could be more profitable to sap into sugar cane than native species. In that case, this idea just died in terms of helping the environment.

This one is fun!

Imagine a large courtyard, that is made from conducting plates. On top of these plates, there is a insulant layer of material. 

You charge the plates with +Q, and keep them charged while the courtyard is in operation.

People wear special shoes, that are have conductive material on them. You charge these shoes with +Q, just to say a number, it actually needs some studying to come up with a good number for the charges. 

When people move on the surface of this courtyard, they should float and slide just as in ice skating.

This is meant as a sport. You can make it more like hockey if you would like.

There is a device that is essentially a like a light bulb, with vacuum inside, Instead of the lamp filament, there are four vanes or leafs that are coated black in one side and metallic silver on the other. 

When light strikes this apparatus, due to the pressure of radiation in the black and silver parts it begins to spin [loosely speaking]. This is a Crookes' radiometer. It you don't get the shivers for a something that spins out of light, you are in the wrong trade.

The idea is attach small and light magnets into the shaft so that when the Crookes' radiometer spins, it generates a small electromagnetic field. Add thousands of this units, and perhaps you've got a beacon that does not require circuitry or solar panels to function.  

It is meant to be used in space.

If you get a syringe real close to the skin, and there is a blood vessel right underneath it, really close, could you make an injection's shot quantum tunnel through the skin barrier, somehow? 

This means, transfer the contents without breaching the skin. 

How can you tell a photon that has bounced off a mirror from a photon that has not? 

Can you tell if a photon exists for half a second or two billion years?

I've often wondered, why doesn't the hyperlinks have a "dead link parameter", that would tell in case a link is broken. If this "is dead" parameter is TRUE, the link is highlighted red or something, and people know before clicking on them. 

Special bots would be designed to keep roaming the web and verifying links, updating the dead parameter.

Maybe this exists to some extent already.

A long time ago, I remember a trick where there was a cell in excel that when clicked would open an Easter's egg. I don't know anymore if that was an urban legend or not.

The fact is, should you hide text in an Excel's spreadsheet, in one a of billion cells, how hard is it for somebody else to find it without knowing its exact location?

Could this "hiding in barren vastness" strategy could be used as a second layer of encrypting?