First a disclaimer: every write-in is unique and reflects the style and preferences of the write-in host as well as the quirks of the write-in environment and preferences of the participants. E.g., libraries tend to be quieter than cafes.
That being said, there are some things that we do at some write-ins in the region, so I will include them in this write-up; and you will at least see them at the Naperville ones I am hosting and the Woodridge ones that Catherine (Cee-Bee) is hosting. Katherine's write-in will also have some of these features; and everyone is welcome to try these at their own write-ins.
Word wars are timed writing sprints where participants see how many words they can write during a preselected timespan. Sometimes there are little prizes. People usually record their starting wordcount before the word war begins and then their ending wordcount when it ends. In our region, we use the shared spreadsheet at naperwrimo.org/wordcount to track these during a write-in. If you don't have wi-fi (e.g., if you are hand-writing your novel), you can ask your neighbor to update your wordcount on the spreadsheet for you.
Hand-writing your novel
Since people typically write more slowly by hand than they can by typing, we allow wordcount inflation for word wars where prizes are concerned. Rule of thumb is, I think, a 1.5 factor.
What should you bring to a write-in?
Whatever you need to write your novel (e.g., laptop, power transformer). You might also consider bringing an extension cord and perhaps a power strip. I always travel with three of each for the Naperville write-ins. Most venues allow snacks, so you can bring some for yourself and/or some to share. Sometimes folks bring their stuffed animals to bear witness to their writing :-)
Vegetables of Achievement
Formerly Fireballs of Achievement, these are paper balls (crumpled paper wrapped in tissue paper) with paper tails. Think of them as ingredients for our novel stone soup (our regional theme). When you hit a 5,000 word boundary in your wordcount, you can get one (if the host brings them to the write-in or if you make your own), stand and call out "Huzzah!" as you toss it into the center of the ring of tables. Everyone else will stop what they're doing to applaud your achievement. You can also throw one of these if you do something significant in your novel (e.g., kill off a character or deal your main character a big set back; or have your main character take a big action).
Box of Doom
This is a container of colored popsicle stick. Each color represents a range of wordcounts. People pick a stick in their desired range and have to try to achieve the wordcount marked on the stick within a 17 minute timeframe. Will you pick a wordcount that dooms you? Winners get a piece of chocolate or a little card, or something like that.
What you do at write-ins
Write-ins are about writing. We will have one or two meet-ups during the month for socializing, but write-ins should focus on writing, not chit chat. This doesn't mean that people won't talk to each other, but the predominant activity is always writing. Sometimes if people get too chatty, that's the perfect time for a word war. :-)
Some write-ins don't have any word wars and people just write; and this is fine (and some people prefer these style of write-ins).