Zucchini
Zucchini is a great vessel for flavor when put it on the grill. As a stand alone grilled vegetable, it’s fine, but with the right tweaks it can turn fantastic.
From the easy “coat it in olive oil and minced garlic” approach to unhealthy and messy cheese-encrusted Parmesan strips, your dad loves grilling with zucchini because he can try something different with it each time.