This has 99.997472% chance of being a waste of your time.

A ballon of Helium floats, because it is less dense than air. Grab a paper and pencil.

First, there is rubber ballon and a mass, tied by a rope. These are dimensioned so that the forces balance, and the hole thing stays at same height. No problem there. 

Then you get a couple of hoses, and two pumps. These pumps have two entries, one out and one in. You cut two holes on both sides of the ballon, to the right and left of the vertical rope, and glue two separate hoses on these holes. Their extremities are dangling. 

The left hose coming from the ballon is fixed on the the output of one of the pumps, that is placed to left and a little under the ballon. 

The hose of the right you fix on the input of the right pump, on the right on the same level as the first pump. Then you get a third hose, and connect the out of the right pump to the in of the left pump. 

This forms a closed loop, and now no Helium can escape, it will just fill or empty the balloon. If you energize the left pump, the ballon fills up, and the mass lifts. If you energize the right pump, the ballon empties and the mass falls down. If you alternate the pumps, the ballon-mass oscillates, albeit slowly, up and down.

Now here is the trick: You would attach a point of the rope to a linear to circular motion system. Then you take the output of the rotating shaft and connect to a generator.

If you made it through here you are a champion.

Would this whole contraption, produce more or less energy than required by the pumps? 

No matter how many times Physics lectures you attended, sometimes you just can't resist the urge to develop a free energy machine. 

Can you reuse the waste heat from your computer? Can it, at least, keep a cup of coffee warm?

Pyramids are loads of fun.

Imagine designing and building an army of solar-powered robots whose sole purpose is to move sand here and there, building the tallest sand pyramid in the world. 

It could be a cone too, it is your show. I'm taking about an artificial mountain that would be seen many miles away. After you are finished, mark it as a tourist attraction and profit.

This is desert idea, I get that a lot. 

Imagine many roaming robots, solar powered. They would shovel the sand or a small area in the desert. Their purpose is to check if the area under the sand, now visible, has gold veins.

They would inspect the area and if that is not found, keep shoving sand, moving the hole from place to place.

Imagine that you have a a doughnut shaped rod, that is has a threaded pattern in the surface. You can think of it as a threaded rod that you bent to a circle. But before bending, you placed a object in its threads. It is a threaded ring, with two bar magnets attached to its surface, one inside the circle and one outside. The ring can rotate freely on the doughnut.

Now, you give a nudge to one of the bar magnets. As it moves it is repelled by the other magnet. Does the ring rotate forever?

Can you generate energy, somehow, from the difference of pressures in the ocean?

It would be something like building a collumn that scoops water in great depths, at high density and pressure, brings this container with water up and makes it flow through a turbine.

I'd pump air to lift the containers I guess.

Can you slow down the fusion processes in the Sun, to buy us more time?

Can you trick ants into moving, and while they are moving, you make them rotate a wheel or something to get energy from them?

I've considering once that ants could be fooled into carrying small charged spheres.

What if instead of all the hassle of producing food, we just injected ourselves with ATP produced in an industrial process.

Would that work?