I might consider investing in some spf clothing if it is in your budget. Also, it can get super hot in an infant carseat attached to a stroller base--especially against the back. As a PP mentioned, you might want to plan on switching up how you carry the baby--in arms, in a carrier, using the stroller. I had a winter baby and my ped gave me the green light on a barrier/mineral sunscreen for my baby at 4 months. I would ask about sunscreen, how many wet diapers to expect in the heat and if ear protection is recommended at your 8 week appt.
Even if it is not something you would normally use, I would put diaper cream on because of the heat. If you have room, pack 3-4 changes of clothes for the baby and a shirt the grown ups could share in a pinch--babies seem to pee or puke on you when unprepared. Also, ziplocs to put any clothes you have to change in. A blanket will turn any soft shaded area into a great place for tummy time even though it seems crazy to pack one for Disney in the summer.
If the mom is nursing, powerade/gatorade may be called for instead of regular water. Consider using bottled water if using formula because 'new' tap water can cause upset tummies in some babies.