If you want to stick with simple bolt-ons w/o tearing into the engine, the best upgrades to start with would be a better flowing intake and exhaust. You can swap in a performance intake manifold, TBI unit, filter/tube assembly, headers/y-pipe, and cat-back exhaust. You can add some power, throttle response, and possibly even better mpg by upgrading the ignition system also (distributor, coil, plugs, and wires). Edelbrock makes some nice performance tbi units and intake manifolds. Accel, Jacob's, Mallory, MSD, etc, all make upgrade ign components for GM HEI systems.
Once you get your bolt-ons done, you can have a chip custom tailored to take full advantage of your engine from companies like JET and Hypertech.