Camera.

Where are you staying.

Can I assume YSP is Yellowstone since you tagged the post as Wyoming?

You should spend at least part of a day in Grand Tetons. Take the Jenny Lake boat and hike to the waterfalls.

You should also walk thru Geyser Loop and maybe all over the Lower Basin. Not just watching Old Faothful from the deck.

Go to Morning Glory out in the Lower Basin. Grand Prismatic Spring in the Middle Basin. Both have amazing colors.

I thought Old Faithful would be a mere tourist attraction. It's truly amazing. The best part is it has a short cycle time.

At least one of everything - short sleeved shirt, long sleeved shirt, shorts, long pants, fleece jacket or sweatshirt. I didn't travel with a rain jacket. You may get summer weather, fall weather and maybe even cold weather - sometimes all in the same day. If you aren't doing hikes, then you probably don't need the boots.

Jacket/fleece/or sweatshirt. You can count on it being chilly at night and quite possibly during the day. Weather can change suddenly, pack to layer up.

Either bring a small cooler and use it to pack in or buy a cheap one when you get there. It is a big park and you don't want to be stuck in a bison road jam and have one of the kids whining they are thirsty or hungry - and I suggest you pack at least car snacks or perhaps lunch each day.

It is likely to be colder than you expect rather than warmer.

Try to find some very lightweight/small plastic type ponchos (you can buy them in small plastic bags - sporting goods stores, fishing stores, etc) - which will help if you get a sudden shower, and can also we used as a "wind" shirt to help keep out the chill. Just have everone carry one in their pocket as insurance against sudden weather changes, which can happen anytime.

The summer thunder showers are more daily weather driven (hot air rises, cold air rushes in, etc) - and usually don't last too long, but you might get a front moving through.

Thanks everyone. Think I will bring fleeces and the rain ponchos seems like that will have us covered. Not packing a cooler rather bringing insulated bags just hope I can find ice or refrigeration. Will bring granola bars and pbjs as emergency food for the road.

Binoculars for animal viewing.

Have an awesome trip!!

Meyer - Sorry neglected to answer you. Three nights in the park - Roosevelt for 2 and OF Inn for 1. Also traveling the Beartooth Highway so doing a night in Red Lodge and one night in Big Sky. Not going to make GT this time around want to focus on Yellowstone. Thanks

I was in Yellowstone last June and I learned my lesson about expecting the worst, we stayed at Lake Village, average temps were supposed to be 40s low, 60s high. It was 28 degrees and three inches of snow and highs in the 40's. Pack warm and bring a decent camera.