Visited Sicily a few years ago and it was great! Although you can eventually OD on old cities with great baroque buildings. Some highlights:
- Agrigento & Segesta with their ruins (didn't make it to Selinunte)
- Zingaro 'National' Park (or 'Natural' Park ?) west of Palermo - nice hiking
- Village of Erice high above the valley on a plateau near Trapani. Also visited Marsala (most definitely try the wine).
- Dome of Monreale above Palermo. Norman Palace in Palermo.
- Port of Cefalu east of Palermo
- Frescoes at the ancient roman villa at Piazza Armerina - do not miss this, really really nice
- Taormina is of course great, and Syracuse city & ruins is also worth it
- Back then it was still possible to hike on Mt Etna & take a cable car up to the top of the skiing area (where of all things we ran into a full fledged Spanish 'estudiantina' band in full attire who even performed a few songs for us in the snow - one of those things that are pretty surreal...). I think however that with the recent eruptions this is probably not possible anymore.

You'll love Sicily, the people & the food are just great. Prepare to eat a lot!!