what to see and eat? Buses ok from Aix to Nice?
I did a day trip from Marseille to Aix a couple of days ago, so I can't answer your transportation question from personal experience. I'd use the most convenient travel method, bus or train. Do be sure you know the exact departure point of the bus if you opt to go that way.
Aix is a very nice strolling-around city. Ask at the Tourist Office about markets. I believe Tue, Thu and Sat are the big days for the happenings right near the T.O., but there may be other markets at other locations. Do seek info before heading out on a market quest, because there are a lot of torn-up streets, including one area where markets used to be held.
The three sights I took in were:
Musee Granet (art), which has a nearby second location, the Granet XXeme, for a loaned collection of early-2oth century art, which is not to be missed unless you like only classical art. There's even a Van Gogh (though I'd guess the critics consider it a more minor one). There's a very interesting English audio guide, at least for the XXeme. Both parts open noon to 5:30, but the hours for Jun, July, Aug and Sep may be expanded. One hopes so.
Cathedral St.-Sauveur (sp?). There are supposed to be tours of the cloisters ever 30 minures (with a break between the 11:30 tour and the 2:00 tour), but no one showed up to give a tour at 10:30, so I don't know how reliable that schedule is. The 11:00 tour went off as scheduled. Tours are in French only and last 15 to 20 minutes. It was worth it to me because I love looking at cloisters. I don't really understand French, but it was obvious that something was being said at the end of the tour about voluntary payments/donations--whether to the docent (surely an unpaid volunteer) or the church, I couldn't say. Not everyone pulled out his wallet, but I handed over 3 euros. The rest of the church is worth a visit as well and is free to enter. Your guidebook will point out the highlights.
The memorial site at Camp des Milles. This was originally an internment camp for enemy aliens early in the war, but it ultimately became a transit camp for French citizens and foreign refugees who were shipped out to Nazi death camps. The facilities remain intact, and they've done an excellent job with the audioguide, which is available in English. Two short films are included, one near the end of the visit that apparently gets into the "how could this have happened?" question and discusses some post-war psychological experiments that delved into that matter. I say "apparnetly" because I ran out of time and couldn't see the film before the site was due to close at 7 PM. I think you'd need two hours to be sure of catchimg both films, or substantially longer if you wanted to read a lot of the informational displays covering (among other things) the rise of the Nazis in Germany, life under the Vichy regime, and the acts of brave people who saved lives of some of the threatened). I'm sorry that I don't know the opening time, but I'm sure it's online, and the T.O. has the info.
I took Bus 4 to and from the site; I picked it up a few blocks from the T.O., which marked the bus stop on my map. I think the somewhat less frequent Bus 14 also goes there, from a different bus stop also pretty close to the T.O. The stop nearest the memorial site is Gare des Milles, then there's a 500' (approx.) walk, well-marked. The bus ride will take at least 20 min., more during rush hour. Visiting the camp is a significant commitment of time and money. I paid 11 euros, including the audioguide, and that probably was with a minor senior discount. But I recommend the visit highly. This was not itself a death camp, so it might be easier to deal with for the more sensitive visitors than a place like Dachau. Yet is is still very moving.
During the time when you're walking through the lower levels of the camp it will be quite cool even on a hot day. If you're wearing a sleeveless T-shirt, you might want another layer.
The timetable of the bus route from Aix to Nice is here (French only). Note that not all journeys operate every day of the week - see the top of each journey column. It's a long ride and the section along the coast into Nice is likely to be congested once you get off the motorway.