Most of the trail is jeep road with a little bit of single track mixed in. It's all relatively flat with no severe climbing. There is not a great deal of shade so be sure to bring plenty of water.
What I enjoy about the trails at Ka'iwi State Scenic Shoreline is the ability to pick your own path and explore a bit. The area is full of trails that crisscross each other allowing you to choose your own route. There are Kiawe bushes in this area so bring a spare tube just in case you pick up a thorn.
If the parking lot at Makapu'u Lighthouse is full you can also park and access the trails from the Sandy Beach parking area.
Clicking on the link above you bring you to Google Maps where directions to the trail are plotted in from Honolulu Airport.
Download the GPX file and use your favorite software to upload trail data to your GPS. Right click on the link and select "Save Link As..." in order to download it.