Here are a few thoughts for you to consider in the area I'm familiar with around Salt Lake. A link for overall info on things to do:
https://www.visitsaltlake.com/things-to-do/
Here are some personal highlights that are free. Free is good.

And also close to I-15. There are other great areas to visit too but many of these are not close to I-15 (Park City is Amazing but 30 minutes or so off I-15, Zions National Park is Incredible and not too far off I-15)
In Salt Lake the biggest tourist draw is the Mormon Temple Square. The Temple building itself is beautiful and there are water features around. Plus, this is probably the best place to see amazing flower gardens for free. Seriously, beautiful flowers. If you want to enter the visitors centers there for more on Utah and Mormon history you can, but can also easily be skipped. This is just a few blocks off I-15 so very easy to get to.
If you like to hike, there is a short hike up to Ensign Peak that offers a beautiful view of the Salt Lake Valley, and again really close to I-15 and downtown Salt Lake City. Here's a link for that:
https://utah.com/hiking/ensign-peak
For us, there's not a lot that will get us to stop between Salt Lake and St. George close to I-15. However, in the St. George area there are some quick, fun and most free or relatively cheap things to do and see. The southern end of Utah has some really pretty red rock areas. As hinted at earlier, Zions National Park is really pretty but a bit off I-15. Kolob Canyon is closer to I-15 and quicker to see. One of our favorite little gems basically right off I-15 is the Red Cliffs Recreation Area. The red rock is beautiful and there's a short little hike along a creek that would likely have water in it in the spring that leads to a little waterfall. Very easy hike. You can go past the waterfall if you wish too and the canyon gradually narrows into more of a slot-type canyon. There is a very small day-pass fee here and it's totally worth it. Here's a link for more info:
https://utah.com/hiking/red-cliffs
So, there's a few highlights. Again, I've tried to focus on close to I-15, easy, and free. There are a lot of other things that have higher entrance fees or a bit more off the freeway path.
If you want any more specifics or other types of points of interest, let me know.