a bunch of various sized plastic/garbage bags to hold wet items
If you do not have these items, I promise you it will rain like crazy and everything you have will be soaked, even if the forecast doesn't have a chance of rain in it.
I would also suggest bringing double things like shoes, socks, jeans and tshirts/sweatshirts because you may get wet and certainly will get dirty crewing and its nice to have clean clothes to change into (let me tell you how fun it isn't to walk around with one wet shoe for hours...) I also suggest bringing several towels, in case you get wet or in case a kind soul with a shower offers you its use!
I didn't see medication on that list, but I would certainly make sure to have your NSAID of choice (advil, tylenol, etc) on hand. I would also bring some type of small first aid kit (band aids, triple antibiotic ointment, etc) just in case.
Also not sure how HITS specifically works, but I would double check with management that food for sale is available. Some rides I have been to offer food outside of official ride meals (generally dinner the night before and/or the night after the ride), but most only provide ride meals. If you are officially volunteering though, that may mean you get fed. Again, I would ask management. Would really suck to be without real food except for dinner!
You mention bringing your phone, but do you have something waterproof to carry it in? If not, double or triple plastic baggie'ing it has worked for me in the past. Def going to want it on you so you can take lots of pictures for us!!