If if have a 305, put in a 350. That's the only "easy" motor swap that will net you more power. Swapping the L31 vortec style heads will net you as much as 30-40 hp, but requires a specific intake to work with your TBI system. That intake has gotten expensive.
383s are somewhat popular. More torque for sure, but you have to buy it or build it, since GM never made one.
Someone will say LS swap, and it's definitely a route to more power with better gaskets and newer everything, but it's not "easy", at least by my definition. All depends on your budget, skillset, and equipment.