Though it always plays second fiddle to Utah, Idaho has been very important in the history of the LDS Church. LDS pioneers built settlements in Idaho as early as 1855, just a few short years after arriving in Utah, and there has been a strong LDS presence in the state ever since, especially in the eastern half of the state. Currently, approximately a quarter of the population of Idaho are members of the LDS church, the highest concentration of anywhere outside Utah. The Idaho Falls temple is the 8th operating temple in the world, and first in the state. It was the first temple to be built with a single central spire, and has a unique design (though the concept of the central spire was used in other temples). It also has a unique location, serving as a backdrop to many majestic photos of the falls that stretch across the Snake River, which runs alongside the temple.
Idaho Falls isn't a large city, but it is the largest in the eastern portion of Idaho, and the second largest metropolitan area in the state, so it's as close to a big city as there is for miles around. In practice, this means a nice mixture between a casual, slower rural feel, while still having most of the amenities one would look for. This combination often finds Idaho Falls listed on "Best Places to Live" lists.
For a smaller town, Idaho Falls has a number of fantastic restaurants. Get a great homemade meal at Smitty's Pancake and Steak House or Dixie's Diner. Or find a hidden treasure at The SnakeBite Restaurant, where you'll find upscale food in a casual environment. Or why not add a little spice to your life at Pachangas. Then cool down with Reed's Dairy, the best ice cream spot in town.
There are a number of fun attractions in the surrounding area, including the Museum of Idaho and the Tautphaus Park Zoo. Or take a walk around the beautiful Idaho Falls Greenbelt, and stare at the man-made wonder of the falls themselves. Take in a drive-in movie at Sky Vu. Then, as you head out of town southward, stop in to Lava Hot Springs to relax for a bit. Idaho Falls is only a little more than two hours from Brigham City, and a worthy addition to a Northern Utah temple tour.