Dubrovnik has 4 official monuments which are included in almost every common city tour : Franciscan monastery, Dominican monastery, Rector's Palace and the Cathedral as MUST SEE when you are in the city.
Apart of these 4 there are many other places you can visit after the guided tour i.e.,church of St. Blas or the Maritime Museum, Ortodox Church or just walk the upper streets and discover their beauty. All of this can be done within 7 hours, and even more than that.
If you like to book the guide please visit their site
Cheers and hope you will enjoy your visit.