Carmona is an easy day trip from Seville, with buses leaving every hour or so from the Prado bus station. It's a perfect fit for one day. While the central square where the bus drops you off is not so picturesque most of the sights to see are in the older portion of the town. You'll first notice the 15th century tower modeled after the Giralda in Seville on your way up the hill to this section of Carmona. The Alameda is a nice stop for a rest, with fountain and a large plaza where you'll often see children playing or folks simply passing time. Upon reaching the older city center you'll pass through the Puerta de Sevilla leading to the more charming portion of Carmona. The tourist office is located as you enter and you can get a map guiding you to the sites. Just to the back of  Plaza de San Fernando is the main market where you can do a little shopping for fruits, vegetables and fresh meat if you arrive in the morning.

Up the hill is the Parador which was first built by the Moors and later renovated and taken over by Rey Don Pedro. Now a hotel, it preserves the traditional patios and houses many antiques. While you may not choose to stay there for the night you can visit for a drink in the restaurant area. If you're lucky a table will be free on the outside terrace. The drinks are a little expensive but it's worth it for the view. Dining here is good but quite expensive as well and you would do better to choose an establishment down the hill a bit if you're on a budget. Close by are the Puerta de Córdoba which was rebuilt according to it's Roman origins. The gothic Iglesia Santa Maria, like so many churches in Andalucia, was built over a mosque and preserves the original minaret.
A trip to Carmona should also include a visit to the Roman necropolis which is at the bottom of the hill. Once on the grounds you can step below into the actual burial rooms, dating from the 2nd century B.C. to the 4th century A.D.. Urns and some frescoes have been preserved and close by there is an amphitheater. Carmona's Museo de la Ciudad (City Museum) also has a number of good exhibits, covering Neolithic, Roman and Islamic periods amongst others.