Yellowstone is a place of adventure but it can also be a place for romance. It depends on what you plan to do at the park as to what you should bring.
There are some very nice restaurants in Yellowstone National Park like the Snow Lodge or the Yellowstone Lake Lodge. You can dress up when you visit these fine restaurants so bring along dress clothes if you would like a fancy evening.
Most of the park is for sight seeing and hiking so your wardrobe should accommodate those activities. Everything you wear should be comfortable and match the weather you are experiencing. Yellowstone can be hot one minute and cold the next so it is good to dress in layers. Bring shorts and t-shirts in the hot summer but also pack some pants, socks, and sweatshirts. Rainproof attire is a must in Yellowstone National Park since it can rain at a moments notice. A water proof raincoat or jacket and an umbrella may be good choices to have along with you. Or a disposable plastic parka is nice since you can fold it up for hiking and it does not weigh much.
Comfortable hiking shoes are most important if you plan on walking anywhere in the park. It always seems that you walk farther then you plan on when in Yellowstone. I also like to have a pair of comfortable sandals to wear when hiking in hot weather. There are some really great comfortable sandals made for hiking in and you can find them at sporting goods stores.
The sun can beat down hard in Yellowstone so a good hat is very important. One that covers your ears and back of the neck since you can burn easy in Yellowstone.
Sun screen can help keep you from burning in the hot sun and insect repellent is necessary on most trails. Sunglasses can help keep your eyes safe but they will also keep you from seeing some of the fabulous Yellowstone colors.
A water bottle and snacks are good for taking along on hikes and a little backpack to put them in is helpful. Do not take a big backpack or a heavy purse since they will just slow you down and make you uncomfortable.
The lakes and rivers are very cold in Yellowstone so you probably do not need a swimsuit. Wading is about all you want to do in that cold water.
Remember your medications and other necessities as you pack for a trip to Yellowstone National Park.