Jumping in here with a more radical suggestion. Assuming your in-laws are from out of country. why not give them a holiday in the greatest of the seven great natural wonders of the world.
There is so much more to the Grand Canyon than just peering over the rim at its granduer.
Fly into LV and next day head for the Grand Canyon South Rim. Before you enter, go see the IMAX movie of the GCNP. This will help you appreciate the GC so much more. Spend a day and a half at the GC, staying in the park if at all possible. If all the adults are able bodied, enjoy some of the shorter walks, and take part in the ranger led programs and walks. Exit out the east entrance, stopping at most of the many overlooks and view points, and stay at Cameron's Trading post.
Then head to the North Rim via US89/US89A. First sign you will come to is Horseshoe Bend. A 20 min walk each way to see one of the many scenic bends the Colorado river carved. Take in some, or all of the amazing sights along the way - Navajo Bridge, Marble Canyon, Lees Ferry (the only place within Glen Canyon where visitors can drive to the Colorado River in over 700 miles of canyon country) and Vermillion Cliffs National Monument. These are all great sights to spend some time and learn their history, especially Lees Ferry, if you drive right to the end. The natural formations all have explanations with them.
This is also a scenic route to Fredonia and onto the North Rim, via Jacobs' Creek. There are also other sites along this route, but these will keep you occupied and give a true appreciation of Grand Canyon Country. The North Rim is at an elevation 1000' higher than the South Rim and it is a very scenic drive. Well worth taking your time over. Your entry ticket to the Grand Canyon is valid for seven days.
Give yourselves time to really absorb the difference between the North and South Rims. Again take part in the ranger programs offered. Hike some of the trails. Go see some of the remains of centuries old native dwellings And be sure to take in both a Sunset and and a Sunrise over the canyon. There is nothing on earth quite like it.
That should take up most of your time, and your visitors will have seen a lot more of Grand Canyon country than most visitors ever get to see.
Lifey
P.S.
Here's a book which will help you understand the GC even more.
