UV filters (like these Hoya HD3 UV Filters) are a must when working in harsh sunlight because they not only block out harmful rays that can damage the coating on your lens, but they guard against dust, oil from your fingerprints, and moisture, that over time, can really destroy your lens and affect its performance or clarity. They might be not-often-remembered items for your kit, but they are very helpful when traveling and working in the outdoors.
Neutral density (ND) filters
Neutral Density Filters or ND Filters (like this Hoya Infrared Neutral Density Filter) are like polarized shades for your lenses, and are best used when light is harsh and you need a few extra stops to not overexpose your shot, especially when you’re wanting to set your camera at a wider aperture or slower shutter speed, which naturally lets more light in. Look for an adjustable ND filter that can add at least ten stops to your lens instead of carrying multiple filters for one job. Again, a piece that is overlooked, but one that increases your usable daylight when shooting.
Finally, Polarizing filters or CPL filters (like this MeFOTOWild Blue Yonder Circular Polarizer Filter) are fantastic to have, not only because they protect against harmful rays, but also because they eliminate those pesky sun flares in your shots, and will add much more color and contrast to your photos. This filter can eliminate time in post-production when you’re having to bump up colors or eliminate sun flares, and will really be helpful on your travels.