Let’s just admit it: magnets are fun. There is something inherently entertaining about playing with invisible forces that satisfies the inner child that still longs for Jedi powers and hoverboards. Therefore, I pounce on any chance to buy cheap magnets, and I never throw a magnet away without trying to find a new use for it. I recommend you do the same.
I think Lego® blocks are just as irresistible as magnets, so combining the two is win-win situation. I insist on high-quality, entertaining magnets on my fridge, and these meet my standards (as usual, my rules for what is entertaining or high quality are exacting and bizarre).
I bought several dozen magnetic “push pins” from Staples when they were on a deep discount. The pushpins are useful enough in their original form, and they’re a cheap source of extra magnets for projects like this. The easiest way to remove the magnets from their plastic, push-pin shaped handles is to crush or smash them somehow. I did it with a hammer on a piece of scrap lumber. Whatever you do, be safe, and be careful not to lose the small magnet in the process.
It just so happens that the push-pin magnets are a precise fit for the cylindrical supports found underneath every group of four Lego® dots. They will stay put snugly with no glue. If you have these exact magnets, then your job is done. The magnets are so low-profile that you can still assemble the blocks as usual, allowing you to expand your creation as you like.
If your magnets aren’t the same size as the ones I used, you’ll need to attach them to the blocks somehow. Office and school supply stores sell self-adhesive magnetic tape that you can cut to match the length of any Lego® piece. For oddly-shaped magnets, try a strong glue (not Elmer’s) or epoxy to attach the magnet to the back.
Equipping one Lego® piece with multiple magnets will result in a stronger magnetic pull, especially if you check to ensure that the magnets all have the same pole (positive or negative) facing out. Magnets have both positive and negative ends, found on the opposite ends of bar magnets like these. Matching charges repel each other, and opposite charges attract each other, so you should be able to figure out which poles match just by playing with them for a moment.
Once you have these skills, you can turn any lightweight object into a magnet overnight. Altoids tins and similar metal containers make great magnets without any adhesive as all and lend themselves easily to new applications.