Oooooo! one of my favourite areas of France. Nay,the world!
I assume you have access to a car while you are there? Nimes itself is great, although quite confusing to drive around, the first time we went drove round in circles for ages til we figured it out...then lots of hidden squares to sit at a cafe and people watch in between sightseeing.
Orange is a nice town just north of Nimes with a great roman amphitheatre worth a daytrip, and it sometimes has plays and things going on. Avignon is definitely worth a visit - lots of papal history (check out the Pope's Palace) and lovely shops views etc and the home of Chateaux Neuf du Pape - a reason to visit in itself (...unless you are t-total )
Just above Orange off the A7/E15 is a place called Pierrelatte which has a cool crocodile farm which is heated by the nearby nuclear power plant. Sounds scarier than it is from both respects.
Directly north of Nimes is a lovely small town called Uzes - nice restaurants under the arches in the main square.
Not far from Nimes is the Pont du Gard a roman viaduct - v. impressive, and if you are into camping, some great sites near here. Great drives up the Gard valley too.
Also Arles, towards the coast is nice with losts of spanish influences - they do great Paella and have an active bull ring and lots of festivals during the summer months.
If you are prepared to travel a bit further north of Aix-en-Provence up the E712 is a small market town called Forcalquier near to Manosque. Lovely place - I think Monday or Wednesday(?)is market day and well worth a trip to get your typical french produce, and some colouful tatt on the side, but get there early as gets REALLY busy. Also great restaurants and souvenir shops if you miss market day.
Hopefully the Lavender fields will still be in full bloom while you are there as really spectacular.
Hope you have a brilliant holiday