Haven driven mine as an N/A for about 5 or 6 years before turboing it, I can tell you there aren't a whole pile of options for making a lot more grunt out of the N/A engine unless you want to spend a lot of money.
Some of the more common upgrades include a tubular intake manifold from a bug-eyed Corolla, these come up for sale pretty often and do make a nice improvement. You could couple that with the likes of a K&N panel filter or a full upgraded filter system of some sort (BMC etc). There are a few exhaust manifold options available aftermarket also.
But where you will likely see the most difference is in weight reduction. You're unlikely to get huge HP numbers from the N/A block, plenty have tried over the years but even getting to the magical 100HP mark has proven difficult and costly for some, but if you couple some small HP gains with a weight reduction you might get more smiles for your efforts.