The Edzná ruins (8 a.m.–5 p.m. daily, US$3) are the nearest major archaeological site from Campeche City [1], and make for an excellent day trip.
Agencies in Campeche City offer van service to Edzná, with or without a guide, which can be convenient if your time is short or you don’t want to risk missing the bus home.
If you’re driving from Campeche City, take Highway 180 east to Chencoyil, then go east on Highway 261 to Cayal, where you turn right and continue to the Edzná site. The 60-kilometer (37.3-mile) drive takes about one hour. Second-class service is available from Campeche. Ask the driver to drop you at the desvío Edzná (turnoff to Edzná); it’s about 200 meters (656 feet) from there to the entrance. The last bus back to Campeche passes the turnoff at 3 or 4 p.m., but double-check with the driver on your way there.
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/the-state-campeche/campeche-city