One challenge for me throughout the hot weather is perspiration brought about by the wristwatch I wear. It's always resulted in beads of sweat dripping from my wrist. This can have an adverse effect on one's skin or the metal or leather the strap is made of. A great alternative, which hasn't caught on in South Africa, is a NATO strap.
NATO straps, no doubt, have a casual yet preppy look, and they blend in wonderfully with summer. Although they were 'first' used on classic dive watches and by soldiers as a cheap, durable means to strap on a watch, it has definitely made forays into classic menswear and fine dressing.
Because of the array of colour straps available this is the perfect accessory to a casual ensemble. Even suited it can work because how it complements your suit is largely dependent on the texture of the suit. However with a casual dress code it is right at home because you get to work it into your ensemble.
I think what's also important is proportion. A NATO strap is not going to work with one of those big face watches. So, keeping it in proportion is important because nothing comes off garish and showy.
This particular model by Daniel Wellington features a gold buckle and is one size fits all. One feature I am averse to is the process of changing the straps. I don't like complicated things, and the process of changing straps seems arduous and painstaking. The Daniel Wellington NATO strap however is described as interchangeable and simply slide through the pins on the Daniel Wellington watch with no tools required.
With a casual ensemble, I like a NATO strap with a polo/ golfer, jeans or chinos, and canvas sneakers or suede shoes. A cotton or linen suit will also fare well with a NATO strap. It's fun, playful and colourful, and this should be evident in your outfit as well.
They come in a variety of colours and meet the dimension of most watches, complement a myriad of belts and shoes, add refinement to the most simplest of watches, and if you search hard enough some brands offer them in leather.