Hack days should be about code. Anything that stops you writing code (or talking about / editing / refactoring code) is friction.
There's two things in any software project that tend to cause huge amounts of friction - certainly during the early stages. One is tooling. Installing compilers takes time. Installing libraries takes time. Configuration takes time. I remember a day at Snowcode last year when we spent literally five hours installing Ruby, various build tools, make files, modules, browser automation components, plug-ins... I don't think I wrote a single line of code that day. It was interesting, and educational, but a hack day should be about building stuff.
The other huge source of friction in software development? Debates. There's X ways of doing something, and you can't make any progress until you've chosen one. Should we allow HTML in the comments? How big do we make the gallery thumbnails? What colour should we paint the bike-shed? On a "real" project, these decisions are made by the product owner - but for something like a hack-day, if you one person in charge of all the design, decision-making and prioritisation, they'll rapidly become a rather frustrated bottleneck.
So - pick a problem that's clearly-defined and well-understood, and solve it using a language that everyone's already got, that doesn't need a compiler, linker or build environment, and that everyone can run just by opening a web browser.
Choose something everyone's played. Bomberman. Tetris. Lemmings. Asteroids. Pac-man. Have a copy of the game on hand - on a laptop, or an emulator, or bring a console, or whatever - so if anyone asks questions, you can just refer to your definitive reference implementation and get back to work. That'll eliminate debate without handing anyone the poisoned chalice of product ownership on a volunteer-based project.
OK, this weekend we were using NodeJS, so the build/run/test cycle involved restarting the node server (which is Ctrl-C, up, enter) - but the guys working on the renderer didn't even need a server. They built a client-side test harness (index.html), and their build and deployment cycle was Ctrl-S, Alt-Tab, F5.
That's low friction. That's walking in off the street, opening up your laptop, pulling the code, and starting to build stuff straight away. And I like that.