Just back from that trip, spent a week sightseeing with a little fishing each day. #12 hoppers worked on the Lamar as Ingwe suggested. That river was fairly busy and the wind was whistling up the valley that day, which made casting a bit more challenging.
On a few other rivers I fished I had best luck with a beadhead black wooly bugger (#10 I think) swung across the current, or better yet, dead drifted through pockets in the riffles. Nothing big to report, but I managed enough fish to keep it fun! You couldn't beat the scenery, for sure.
One thing I was surprised at was the number of people in the park, way busier than I thought it would be for mid September. The campgrounds in the park were full virtually every night. Some afternoons on the Madison you'd have somebody fishing every 100 yards, though I didn't see a lot of fish hooked.
