Visit Yellowstone & Grand Teton National Parks!

If you are planning a trip to the Yellowstone National Park (YNP) and the Grand Teton National Park (GTNP), below are some suggestions & tips.

Best time to visit. July & August are the most popular months to visit YNP & GTNP. Second half of August is the ideal time frame, IMO.

This is when the summer crowds reduce (since schools would have started in many parts of the US) and the weather is still pleasant.

Drive vs Fly: Driving is a potential option if you are located in one of the nearby US states. Driving vs flying is a personal choice. Account for the fact that there's a good amount of driving even after you reach the YNP/GTNP.
Best airports to fly into:

If your goal is to minimize driving, consider flying into Jackson (or Idaho Falls) and flying out of Bozeman (or vice versa).
If cost is a priority, consider flying round trip to Bozeman or to Idaho Falls.

The most convenient thing is to stay within YNP & GTNP. Both cheaper & expensive options are available. Camping or staying in a tent-cabin or a cabin are typically cheaper than staying in a premium room within the park. Based on our experience, it isn't very hard to find premium rooms within the park even at the last minute.
Check this link to pick your choice of YNP Lodges. Use this link to check availability. Contact me if you'd like to get a discount on some of the lodges inside YNP.
Check this link to pick your choice of GTNP lodges. Contact me if you'd like to get a discount on some of the lodges inside GTNP.

This could potentially be cheaper than the premium rooms within the park. But, it does add additional driving time each day.
For GTNP, stay in Jackson, WY.
For YNP, stay in West Yellowstone, MT and/or Gardiner, MT. Another choice could be Cooke City, WY

Plan on at least 1 full day of sightseeing in GTNP
Plan on at least 3 full days of sightseeing in YNP
Ideally, consider staying 1 night in GTNP; 2 nights in the lower loop and 1+ night in the upper loop. And yes, that means that you have to change hotels multiple times :(
This makes it a total of 4 full-days for both the parts. This is without including travel time, and assumes seeing only the key highlights.
If you plan on taking the off-beaten path and/or long hikes, you'd need to plan on staying longer.

If you want to stay in one location inside the YNP, Canyon is the most centrally located lodging option but it will entail longer drives to get to other parts of the park.
It's nice to have at least one night at Old Faithful as it allows you to tour the geyser basins before/after the crowds of day trips and tour buses.
Before you finalize accommodations for YNP, get familiar with the overall map.

Google Maps: Download maps of the area before your trip
Install the Gypsy Guide - it's an awesome app. It's like having a private guide within your car. It plays automatically - stories, local tips and directions as you drive, so you don’t miss a thing. It offers a great value - all the benefits of a guided tour, and the freedom to drive yourself. It even works offline - download over wifi, and tour offline; no data, cellular or wifi required when using it.

Get up early! Be up and out by 8 am and you will not have a problem with parking or crowds.
Respect the animals and their space. Do not approach them. And absolutely never turn your back on an animal to snap a selfie and always be alert.
Everyone should be cautious but if traveling with children, be especially careful around the hydro thermal areas. It is NOT a playground.
Always wear layers. The weather gets cold early AM & late PM. In summer, the temperate gets pretty high between 11am-6pm or so. Also, the weather can and does change often especially when hiking in high elevations.
You will be walking quite a bit within YNP. Wear comfortable shoes. Walking shoes could be fine if you plan to visit just the popular attractions. Hiking shoes are a must if you plan to hike outside of the popular attractions.
Always plan extra time for driving. Yellowstone is HUGE and is very busy in the summer season.
Have some snacks & beverages in the car so you don't have to leave what you are doing for food. Not good to have anyone "hangry"
If you are taking kids along, make sure that you take their favorite car seats - since you will be driving a lot
Essentials to carry include hats, sun screen, sun glasses, camera, binoculars, phone-chargers and phone-cables for the car.
You will have no cell service & very limited wifi connectivity (if at all). Download maps, itinerary, and any other necessary information beforehand.

Some context - our group had one grand-parent (who was comfortable walking on pathways), two adults, and two kids (7-10 years old). All of the group members were vegetarian.
The thermal areas got everyone excited. Old Faithful, Mammoth, Grand Prismatic and more.
We were able to see Bison in Lamar Valley, Black Bear in Hayden Valley, Elk in Mammoth, Pronghorn in Gardiner, and some chipmunks :). Everyone in the group totally enjoyed this experience.
The Gypsy Guide App was such a boon! My kids got so immersed in the details from the app, and we had some good discussions about the Physics/Chemistry/Botany of the area :)
It was great that all popular attractions had boardwalks or a reasonably flat pathways. Made it easy for the grand parent to walk comfortably. Most popular attractions are accessible with a wheelchair/stroller
We were able to find decent vegetarian meals at most of the YNP & GTNP lodges (inside the parks) - though, we did get bored with the limited choices after a few days. Luckily, we found a good vegetarian/vegan-friendly cafe (Wonderland Cafe in Gardiner)
The kids enjoyed the junior ranger programs at both GTNP & YNP
Switching lodges wasn't not as much of a hassle as we imagined (it did take some planning during the initial packing)
Overall, it was a great family vacation!

Seattle Desi