spandex/lycra shorts or thighs can really make a difference , if you are one of those people who are accostumed already to riding in those, i would recommend to have a pair, they are not heavy or chunky at all, but you can also opt for other things, many folks doing long tours that dont have love for spandex they usually wear some kind of khaki shorts. It is a light fabric, but at the same time it offers good resistence. I would not opt for leather, because it could create some chaffing issues, unless they are made of some super supple, broken in, leather. but anyways leather gets hot with direct sun light and doesnt breath very well.
Basicly get some kind of hiking shorts, and then some seamless briefs and a seat that fits you.