We're attending a wedding in Hyeres at the end of October and are looking for wedding gift suggestions for the French couple.

Lucky you! Hyères is a great place to be. I remember seeing a gift shop tht had a display of wedding gifts--can you find out what the couple would like and buy something on site? Crystal is always nice, and a piece of Moustiers faience would be a fine gift.

First find out if the couple have a liste de mariage, that would make your life easier. If not, try to think of something from your country. Without knowing your budget, actual suggestion are difficult to make.

Although this might sound crass, money is often the preferred gift at French weddings, at least if there is no wedding list. I've been to about ten weddings in France, and have given a cheque at more than half of them. (It's not my preferred gift but it's pretty much the norm among young French people these days).
Most of our French friends had a box (like a mailbox) for cheques, somewhere on display, at their wedding reception.
I'd probably ask the couple if there's a wedding list and, if not, assume that they want cash.