You need to track unique objects in an 8x8 grid of slots? How many unique objects?
If the total number of objects is 112 or less, you could give each one a unique I2C address.
You'd add 4 circular electrical contacts in a "bulls-eye" configuration on the top and bottom of each object so that all the I2C pins would connect regardless of orientation. All objects in the same stack would be connected together on that stack's I2C bus.
You'd have an 8x8 multiplexer to connect to the I2C bus for each of your 64 grid positions in turn, and poll that bus for each of the possible I2C addresses. (Send a ping for each address and look for
Alternately, you could give every object a unique weight, varied by a power-of-two amount. (first object weighs 1000 grams. Second weighs 1001 grams (1000+2^0)) third object weighs 1002 grams (1000+2^1). Fourth object weighs 1004 grams (1000+2^2). Etc.)
Then you'd put a scale on each pad. Weigh the objects, modulo 1000 grams. That result, in binary, would give you flags for every object on that pad.
Or 2 circular electrical contacts on the top and bottom of each object, with resistors that have unique power-of-two resistance on each one, as Juraj suggests in their comment. Measure the resistance and you can figure out which set of objects is present.
The math would be simpler if you rigged the objects so that they made a series circuit through all objects present (put a normally closed switch on the top that shorts the top contacts of that object through its resistor if there's no object on top of it. If there is an object, the switch would open, and the objects on top would close the circuit.)