Some other ingredient options:
Chicken.... dust in paprika and partially cook in pan and remove to be added back after rice. They will finish in the paella.
Chickpeas.... take on the flavor of the dish and add bulk
Tomato wedges..... I like to use canned tomatoes, as they don't really need to cook.
Mussels... inexpensive alternative to clams, but add near the end, as they cook quickly