...and also should mention that the International is the weekend right before Breaux Bridge, so in one long week(2 weekends) you can do BOTH festivals.
It really hasn't gotten too hot to be outdoors...and we always bounce to at least 1 day of jazzfest over in New Orleans (same weekends).
I've got to second the vote for Festival Internationale, although it's been a few years for me. A variety of music (including lots of Cajun and zydeco), art galleries are open. Doesn't feel too crowded because it's spread out accross the City of Lafayette. Stop by Vermillionville for some history.
Is the crafts store in St. Martinville still in business? Nice place to visit down by Bayou Teche.
If you're in SW La during Mardi Gras week, it sounds like the event in Iota is more to your liking. I tried to squeeze in as many places as I could last year, From Church Point to Eunice to, geez, can't even remember them all. What stood out about Iota, was all the local crafters, home made cooking, and an active effort to see that the 'party atmosphere' you're reffering to, doesn't happen there. Comparatively speaking, it's pretty small, but quite focused. The one thing I did find kind of funny though: right after the MC gave a speech about keeping their event family oriented, all things cajun, and traditional, the next band up was an in your face zydeco band. It's the little things that make me smile
Labor Day weekend brings the SW LA Zydeco Music Festival in Plaisance. Hard to beat that, but it is primarily focused on the music. If yer internal organs can take 12 hours of double-kick drum, and your feet can stand 1/2 a day of stellar bass grooves, then you might consider it. Plus you can wheel in your cooler full of whatever you like.
That same weekend brings the Shrimp and Petroleum Festival in Morgan City. It's got music, but that's just one of the many things to see and do. It's very laid back. The drive down Hwy 90 to Morgan City makes for a nice departure from the "prairie" festivals. And who doesn't like shrimp!?
Last year I missed Plaisance ---our flight into New orleans on Aug 31 was luckily cancelled!- (and we didn't become tourist-victims of katrina)... but I agree it is a beautiful day-long festival, and on the Monday following is the annual labor Day Boozoo fest in Lake Charles, a great lineup of music, food and family-oriented fun. Big Hat Fields has his big free bbq on the Sunday in between if I remember correctly.
But beware- it can be BLAZING HOT down there that time of year... often more than a Northerner can handle,
But I have great memories of sitting outside in 100+ degree heat with not a breeze anywhere, and humidity around 99%- sweat dripping from every pore while gnawing on a gigantic fried turkey wing (size of a baseball bat) and the grease drippings merging with the sweat.