I've always thought that there could be a perpetual motion device using magnets. Long ago I purchased several N52 magnets to test out some ideas, None of them worked, life happened and never got to test out the most promising ideas. These magnets are now mostly unusable at this point.
An idea of mine that I didn't get to test:
Using stationary magnets that interact with magnets that are in motion and a material that blocks magnetic force that also moves to block the force until the magnets are close enough.
There is potential. Position magnets and non-magnetic materials in a way to act as a piston (like in an internal combustion engine), linearly (like a rail) and on a circular pattern (like a cam shaft). The pistons move the magnet laced cam shaft which also assists the circular movement of the rod attached to the piston. A circular disk attached to the cam shaft would move the magnetic force-blocking material in place to each of the pistons. When the piston is at its closest to the bottom of the cylinder, the force-blocking material will move to allow the stationary magnet mounted at the bottom of piston cylinder and the magnet that's mounted on the piston to repel each other. Then the force-blocking material will move back into position to block the repellant force. This would repeat in each of the cylinders.
The cam shaft should be attached to bearings, wheels, piston rods and 4 circular disks (in a 2 cylinder setup) and interact with 2 linear ground mounted devices which consist of magnets and coils. Coils are used to produce electricity using one of the circular disks which will have magnets of alternating poles mounted on it. The linear magnet device could be used to produce onboard electricity by using coils on the 2nd circular disk. Or could be used to attract/repel magnets mounted to the 2nd circular disk.
This device would not only be ran solely on magnets using perpetual motion, but should also produce a little charge. Would probably be expensive to build and then need to be modified several times to get any positive results. I'd expect that the "positive" results would be disappointing considering the R and D time as well as the cost to build the device.