It's a matter of air resistance. The more surface area something has, the more drag it will create as it moves through the air, which will slow it down more. To make something that is heavier than a feather yet generates more drag force than its weight, it would need to have a tremendous amount of surface area to create air resistance.

As for an example, imagine a rock attached to a large parachute. The parachute will generate so much air resistance that it floats down, even with the rock.

Make a parachute for it!

It's fun to make parachutes - take a square of plastic wrap and tie a string to each corner.* Then tie the other ends of the 4 strings to something like a large paper clip. In elementary school science class, we had paper cut-outs of a 'gingerbread person' - 4 of them on 1 piece of paper. We put a regular paper clip on the paper person's waist and taped 2 strings to each of their shoulders. Then we held up the parachute and spread it out and let go, to watch it fall.

*The strings were about the length of 1 side of the square of plastic wrap - maybe a little over 12 inches.

Here's what I found with a Google Search for 'biggest parachute':
The new parachute opens to a diameter of 52 feet, making it twice the size of any parachute ever flown beyond Earth. To test the parachute, which was built by Pioneer Aerospace, NASA brought it into the world's largest wind tunnel, located at NASA Ames Research Center in Moffett Field, California, Jul 1, 2009.

NASA Builds World's Largest Space Parachute for Martian Landing.
space parachute

Deceleration is just a way of saying acceleration in the direction opposite the direction of motion, so making a heavy object decelerate faster means making it accelerate backward faster. The acceleration a of an object of mass m is related to the net force acting on the object F, as given by a rearrangement of Newton's Second Law:
to a = F/m.

Note that this is the net force, meaning the sum of all of the forces, those acting in the same direction as the motion of the object and those acting opposite the direction of motion (see not at bottom). To decelerate, meaning a < 0, the net force must be backward, F < 0 (using the convention that F > 0 acts in the direction of motion).

So, to make an object decelerate at all the force acting backward on it must be greater than whatever forces are acting to drive it forward. Since a heavy object has a greater mass than a feather (m_heavy > m_feather), making the heavy object decelerate faster means the force on the heavy object must be more negative than that on the feather.

Assume the heavy object and the feather are simply falling through the air. In this case the only downward force is that produced by gravity, so positive a comes from gravity. However, air produces aerodynamic drag force on objects moving through it. Drag opposes motion through air, so here this force acts in the upward direction, so decreasing a comes from drag. Because they are acting in opposite directions, adding these forces reduces the value of a.

Thus, to achieve a < 0 requires increasing the drag force enough that it is larger than the force of gravity. The drag equation reveals that increasing area is one way to increase drag force. This is exactly the working principle of parachutes.

Other factors are the shape and surface roughness of the object. A heavy object that catches more air will have a greater drag force than one that cuts through the air.

So, to sum all of this up, to make a heavy object decelerate faster than a feather requires increasing the drag force on the heavy object by enough to overcome the accelerating force of gravity. One easy way to do this is to add a parachute to the heavy object. Note that this only covered passively falling objects; any upward force on the falling heavy object will decrease its acceleration. Some methods which seem outside of the spirit of the question would be attaching solid rocket boosters, hot air balloons, or propellers to push/pull the heavy object upward against gravity.

[As a bit of background beyond 5th grade, realize that force, acceleration, and velocity are vector quantities, meaning they have both a size and a direction. When adding (or subtracting, multiplying, etc.) vector quantities, the direction must also be considered. For this question it is sufficient to know that vectors with opposite directions have opposite signs and adding them produces a vector with a smaller size, like adding positive and negative numbers, with the direction of the vector depending on the sign.]

When objects are falling, their weight determines how fast they will go down. That means the heavier object will fall faster than the feather. However, both objects will meet air resistance, which makes them fall slower. The air resistance is proportional to the speed of the object because if it’s going faster, it’s going to bump into more air molecules, slowing it down.

It’s not really possible to make the heavier object decelerate faster, so it will hit the ground before the feather no matter what.