Check out Crutchfield to get speaker ideas. They're helpful especially for getting speakers that will fit the depth of the front doors. With teh stock headunit, be sure to get a speaker that has a high sensitivity rating, measured in db. The higher the number, the less power they need to generate volume - they'll typically work better with low powered factory head units.
I wanted to keep the stock look in my 89 Prelude but wanted to upgrade. I used the headunit from a 97 Accord EX with CD changer controls, added a CD changer in the trunk, and speakers all around. The biggest difference came from the amplified Bazooka tube in the trunk. If you want some base that's not a lot of hassle, I recommend checking out a self powered, enclosed woofer like this. I got the 8" woofers. Simple to install. It runs off of speaker line input and can be set to mono or stereo. A power lead was the only other thing I needed. Some require a signal lead to turn on the amp but mine has an auto function that turns it on when it detects a signal from the speakers.
