I think one day volunteering is not offered because it takes at least 2-3 days to orientate a person, let them know how things are done, what needs to be achieved, give a briefing, etc. In one day a person will not be able to achieve much and possibly will take some valuable time from people working on those projects. Usually they are spread very thinly anyway and their time is precious. Possibly you can identify one of the places/projects you would be donating to and ask them if they can use you for a day for anything at all: maybe reading for kids or doing some art. But I doubt that you would find anyone who offers one day volunteering. I stand corrected of course.
If you are thinking about bringing some donations and have a choice, do bring the money and purchase everything yourself here. There are plenty of supermarkets here and you can get any stationery, books, etc. This will save you some space in your luggage and also you will avoid possible hassle with customs and officially everything is subject to duties and taxes, donations or not. Clothes has some strict rules, apparently second hand underware is not allowed if it is allowed at all. There are some experienced people here who will shed some more light.
Probably, again if you have a choice, once you identify the project, ask them what is needed the most. Maybe it is mosquito nets or sanitary ware, for example. Or maybe they have a program (like the project that we support) and your donation (money) can go towards this program - maybe sponsor a child, or feeding program, etc.
Unfortunately, many people came across the situation whereby their donations are misused, this can happen to toys, stationery and money alike. If you come across the wrong people, they will misuse anything. So time and time again advice is given here to deal only with trusted projects. I will be happy to tell you the one we support, it is in the FAQ list, or some posters here participate in some as well. I am sad to say, but do not necessarily trust what you might find on Internet as projects looking for donations, etc, there are many cons out there.
Tijara Beach Hotel