- Rain boots.
You can buy them here, but if your feet are very large it will be hard to find them in your size. I have some very light weight ones from The North Face that I love love love. This is particularly important for Agriculture PCVs, but they come in handy for most anyone at some point.
- Umbrella -- compact, sturdy.
This is super convenient in that in between period where it's not worth bringing your rain coat everywhere because the rainy season isn't quite over or hasn't quite started yet.
- Bathrobe, or something similar.
You will definitely be living with a host family some or all of your time here. Carting your toiletries, towel, and clothing back and forth in what may be fairly public areas is a pain. Wearing just a towel probably won't be culturally appropriate. I had one sent to me.
- Laptop and/or External Hard Drive, Camera
I know PCVs without them, but not many. They are nice for storing photos, playing entertainment, researching things online (you can buy a wireless internet stick here), and keeping in touch with folks at home.
- Quickdry -- towel, shirts, pants
You aren't camping for two years, but you probably will be line-drying all of your clothing. My jeans often take up to four days to dry, so it's nice to have a few items of clothing that can reliably be clean and dry in a shorter timeframe.
Useful in power outs and for those PCVs with a latrine.
Most of Guatemala is fairly high elevation and is not a hot, tropical climate. It's easier to buy tee shirts down here if you end up in a hot site than it is to find a good coat, sweater, or long thermals if you end up in a cold one.
In general, know you can buy just about everything you need down here. There are loads of Pacas around which sell second hand stuff from the U.S., and unless you have specific brand loyalties all kinds of toiletries are available at the larger grocery stores in hubs around the country. If you are particular about how something fits or are a hard-to-find size, set aside extras at home that can be shipped or brought to you down the line when whatever you bring along wears out.