I have been to Isla Canela and Cristina and was not much impressed.
Ayamonte is a nice little town. Beyond that there are little towns all the way to Huelva and Seville and they are not in my opinion that great. Ayamonte does have some reasonable Tapas Bars. Bear in mind that many Spanish Restaurante and Tapas Bars in small town close after 15:00 (local time) for Siesta, although in Ayamonte the main ones in the centre do stay open most of the day..
One hire company that does let you take cars into Spain without any any additional charges is Luzcar. - http://bit.ly/d5Z4h3
