Welcome to the forum!
An L28ET swap would be the most cost effective way to get "more power", though it may or may not be something you want to get yourself into (depends on your mechanical / electrical level).
Second up would be a basic upgrade: Exhaust header, cam, ignition, MegaSquirt (a lil bit more advanced), maybe some intake work, head work (decking, valves, etc).
More advanced would be a complete tear down of the motor, bore it (stroking would be more extreme), do some heavy head work, etc.
It's really all up to you...
You could get a good start by reading this: