Even less simple: hang a weight on a string. Hold it still. This is difficult, so maybe hang it from a nail or off a board. There's probably still a little swinging back and forth. So either wait (it'll come to a halt eventually, but who says scientists are always patient?!) or get a large (larger than your weight) cup or bucket of water and bring that up underneath the weight. Make sure the weight is made of something that doesn't float if you use this approach! Once the weight comes to a halt, the string gives you a line which points up and down. The weight is the 'down' side of the line.
Now for getting subtle ... which also explains why the earth isn't exactly a sphere.
In keeping our method simple, we assumed that the only thing acting on the weight was gravity. Since we think of gravity as being the thing which provides weight, that seems reasonable. And it isn't too bad. It's just wrong in detail. What we really need to do is consider all the forces which act on the blob (let's call it something else so that we don't confuse gravity/weight with up/down). Gravity from the earth is the main force, which is why the earth is nearly a sphere.
But the earth is also rotating. That means in addition to the force towards the center of the earth, there's a force pushing the blob a little to the side if you're not at the equator or poles. For another wonderful example of my artwork, not to scale and not showing the stuff holding up the blob:
But there's another aspect to this. The deflection of true down from what it would be if the earth didn't rotate means the earth can't be a perfect sphere. In particular, it has to be a little squashed at the poles -- smaller distance from the center of earth to the poles than from center of earth to equator. So the earth's polar radius is only 6351 km, while the equatorial radius is 6378 km (about, in both cases). Try drawing this accurately to scale. If you succeed, please send me a copy. I don't think we'll be able to tell the difference from a perfect sphere by eye.
The fact that the earth is squashed along the rotation axis, an oblate spheroid, was a controversial matter for a many years in the early 1700s. Newton argued for this. But Cassini, who was an excellent scientist as well, and more of an observationalist (he discovered Cassini's Division in the rings around Saturn) argued that the earth was narrower at the equator, a prolate spheroid. It wasn't resolved until an observational expedition in 1738 by Maupertuis and Clairaut. The hard core math types should take a look at Ellipsoidal Figures of Equilibrium by S. Chandrasekhar.