It can be quite rainy
The people in Thonon don't dress to impress. It's definitely more casual & sporty than other French cities. Shorts and flip flops will make you stand out as a tourist. In the summer most women wear skirts or lighter weight pants and/or cropped capri style pants.
Raingear is a must and bring an umbrella too. The pharamacies are well stocked and very helpful. They're closed on Sundays and at lunch time though so that can be a pain. A lot of American visitors ask me for ibuprofen and I haven't found an equivelent here so maybe bring that. Also vitamins are expensive and highly underdosed so travelling with that is smart. Also maybe Alka Seltzer ! I've never found it here. All the local grocery stores have on the spot digital processing now. The lakeside beach here is nice but the water is too cold for swimming outside of the months of mid June, July and August. Bring jelly shoes or flip flops if you're thinking of wading in because the rocks in the water will hurt your feet. There is a ton of sport activity here and around the city so it all depends on what your interest is!