Rain jacket - I probably wouldn't - on a 90 degree day, unless you expect the temps to drop significantly if it does rain, getting a little wet could be very nice. I recall riding in the rain when I lived back east as being refreshing in the summer time. If its a passing shower, you probably won't stay wet long.

I usually carry one tube plus, one or two co2's and a patch kit. I have a very tiny pump in addition to CO2 - it won't get my tire up to 110psi, but will give me enough to limp by with in a pinch.

Do not take your I-pod to a charity ride - especially as it will be your first one. These events tend to be crowded and chaotic and you will want to have every wit you have available to keep yourself and everyone around you safe. If it is a large event it is unlikely that you will ever find yourself alone even if you are not setting out to ride with anyone else..... In any case, it is just impolite to use one - it makes everyone around you nervous.