Calanques & Cassis by boat from Marseilles or by road and then boat.
Perched villages of the Luberon.
Aix en Provence.
Arles, Les Baux & Saint Remy de Provence.
Trip to Chateau d'if in Harbour (weather dependent). Prison featured in fictional film; 'The Count of Monte Cristo'. (Not sure if that counts as a day trip)
Check out this website: http://www.viator.com/destProdSearch.jspa?country=51®ion=485&activities_checkbox=from&from_dd=10&from_yyyymm=7+2011&to_dd=21&to_yyyymm=7+2011&go=Go&from_mm=7&from_yyyy=2011&to_mm=7&to_yyyy=2011§ion=ttd&destinationID=485&categoryName=&activities=from over 8 years ago
As Hilary's answer, I strongly recommend you to visit Calanques & Cassis. You can go to the the nearest calanques from centre of Marseille by bus N28. I went there in last Christmas and it was so impressive. Or you can go there by boat from the habour of Marseil. There are many tours to choice. over 8 years ago