I'm designing an experiment to test the effect of surface area of a catalyst on the activation energy of hydrogen peroxide decomposition. However, I don't think I can use a powdered catalyst because determining its surface area would be too complex (specific surface area is beyond the scope of my course). So, I proposed using a potato as a catalyst because I can get a chunk of it and split it into smaller and smaller pieces every time I want to increase surface area but keep concentration constant. However, my chemistry teacher isn't a fan of the potato because it deals with biology as well. So I'm wondering if there is a heterogeneous catalyst for hydrogen peroxide that I can get in chunks and reasonably split it into smaller pieces? Preferably, it should be at least somewhat common to find in laboratories.
I think your question needs refinement.
Activation Energy is not affected by the surface area of the catalyst. The catalyst is providing a different mechanism to reach the final product than without it. It does this when particles of the reactants interact with the particles on the surface of the catalyst (since this is a heterogeneous catalyst).
The reason the reaction would go much faster if the heterogenous catalyst had more surface area is because of more particle-particle interactions able to take place between the catalyst and the reaction. This would be similar to why reacting a piece of iron in hydrochloric acid would go much slower than if the same sized iron were cut into 1000 pieces and placed into the same solution. The higher chance of particles to interact with each other, more likely they'll have the activation energy to form products.
A possible answer and improvement to your experiment:
If your goal is to determine how activation energy is affected by surface area, then performing the hydrogen peroxide reaction within a coffee cup calorimeter would be great. You could measure out the same amount of water for the calorimeter, use the same mass of reactants, and measure the temperature changed caused for each surface area tested. If activation energy is affected, the temperature change in the water of the coffee cup would be different.
Manganese metal suggested by @EdV is a good choice. A pricier option would be silver metal. It is very good at decomposing hydrogen peroxide, but at a higher price.