Yeah, you should take a pair of shorts. But you should also take sweaters and a coat.

A coat! How sad.

Casual, comfortable, and reasonably sturdy clothes are the way to go, a la the kind of stuff you see in the REI, Eddie Bauer and LL Bean catalogues. Those hiking pants that zip off to make shorts can be a really good idea. Re being able to enjoy sunshine but also get warm for cooler nights/higher elevations, plan on layering. E.g. your "coat" can be a light fleece jacket or windbreaker if you have a sweater you could layer underneath if necessary. Definitely bring a pair of lightweight hiking shoes/boots. And of course your camera, because it is GORGEOUS where you are going!

It can actually snow there in late August. Rare, but possible. (Highly unlikely with this year's weather, of course.) Take a coat or a fleece jacket just in case -- otherwise you might end up buying one there.

Cold weather during my August vacation, so sad! Thank goodness I will be in Vegas at the tail end to warm up at the pool :D

I was there during the day in June (last year) and it was warm enough for shorts (I wore light-weight capris, short sleeved t-shirt and sandals), but weather can be fickle especially when you're in high elevations. Clothing as mudpuppie and bearwife suggested is spot on. Layers layers layers!

(Where's that edit window?)

Here's a link to my blog for the day we spent in Yellowstone.

Awesome! Thanks for sharing!

You can borrow some shorts from me if you want.
I think capris would be good for the in-between weather.
And lightweight track pants and thin long sleeved tees are good for hiking in, since then the sticks don't scrape you and the bugs can't get you.
I like cardigans for layers and zip up fleeces instead of pullovers. I am more likely to actual layer and de-layer if it is easy.

Horseback riding means sturdy shoes with a heel.

You can also make do with good sneakers in a pinch.

I was there in late July several years ago. It was hot, especially in the low-lying areas where a lot of the tourist points are. You will want shorts and t-shirts and light overshirts and definitely a hat and sunglasses, balanced with long pants and sweaters. Old pairs of khakis or those synthetic travel pants are pretty good for hiking, but I actually just wore shorts on all my hikes and it was fine.

A lot depends on whether you're camping and if you're camping, is it more RV-style or more roughing-it tent style. If you're not going to be in the backcountry or spending overnights up high, you are probably not going to need a heavy coat. A light windbreaker/jacket plus a wool sweater would do. I took gloves on my trip but never needed to wear them. The only place I saw snow was in the Tetons at 11,000 feet, and even then it was super warm feeling even though snow was on the ground, and I was wearing shorts.

Especially because it sounds like you're not doing a heavy-duty outdoors-type experience, I think you'll be fine with the same kind/range of vacation clothes you'd take on an early fall vacation in the Northeast. Make sure the stuff you take to wear hiking isn't your favorite, super cute stuff, because you'll be sitting on rocks and brushing by resiny pines and getting sweaty and dirty in them. But also do take a cute sundress and a nice pair of sandals/shoes. Like other National Parks, Yellowstone has a couple nicer restaurants in the hotels and lodges, as well as outside the park nearby, and you might feel like dressing up.

No camping! We're staying in a string of Best Westerns as we travel. I do believe we'll be visiting some nicer restaurants and events (I saw something in the itinerary about a rodeo? hurray!).

The reason why heels are better when riding is because one's foot might slide through the stirrup and get caught.

Oh, I'm so jealous, of both Yellowstone and Vegas... Have a great time!

We were thinking of going up to Maine for a few days later this month to visit Jon's family. We'll see... it would be nice to get out of the city for a bit, too.

Comfortable shoes. Comfortable, easy-care clothes. A few things that can be layered, like tshirt+ light sweater + fleece, and at least 1 pr. medium-weight pants, possibly jeans. If it gets cold, having a hat and socks makes a big difference, without taking up a lot of space. Comfortable, sturdy pants for horseback riding, and sturdy shoes. Mostly, when I travel, I make sure everything looks okay together, so it's no problem if the turquoise teeshirt get a spill on it. Yellowstone is fun.

Pips, if you come to Maine, meetup is an excellent plan.

Cool... definitely.