That sounds like a great race! Lots of fun!
As far as things to take or store. Pare down you keys to a single door key. You may need ID to pick up your race number. If you can pick up your number before the race, all of your ID info will be on your registration associated with your number and you may not need any further ID.
A little cash is always a good idea and of course your bus pass.
I do not recommend putting anything around your neck! I have visions of decapitation
. There are waist belts made for just that sort of thing. You can tie your key into your shoe laces (practice first, but I do this all the time.) I also have a couple pairs of shorts with smallish zipper pockets at the back of the short, just under the waistband. It is just the right size for an ID, credit card and/or bus pass. One of the pairs is a Nike, not sure who made the other one.
Races will often have a spot to stash extra clothes or a satchel. You might contact the race director ahead of time and ask. In smaller races it is just at the registration table. Don't leave anything valuable, but change of shoes, or a fresh t-shirt always makes the trip back home a little more comfortable.
There should be lots of water, and snacks available after the race so you shouldn't need to worry about that.