The most serious advice I would say is really make sure you understand what the charity actually wants and will use.
My Realistic Working from Home Day (as a Programmer) - Tech & Coding
I helped a homeless charity once, took me 10 minutes tweaking a server config. It wasn't cool, or flash, but it did the job and they were happy.
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Don't do something just for the sake of being cool.
That said there are lots of groups that try to offer technical help to charities. It doesn't need to be a weekend event.
(Disclaimer: I haven't tried all of these, just heard of them)
Having done a social innovation camp weekend, I would say that it's easy to get ppl for a weekend of cool fun.
Getting people to commit afterwards for the long build-up and support is harder. Unfortunately, this is often much more important. Most of the groups from the weekend I went to have since floundered.
That said, good on you and all contributers to this thread for wanting to help.