If you do Horsetooth, you'll notice most folks do the standard route to the north summit, but I believe the true highpoint is located on the south summit. There's a climber's trail leading to the base on the east side of the rock, and it's a class 3/4ish route to the top. I was up there two weeks ago, and happy to have a guy spot me on the downclimb. Horsetooth is famously featured on the Fat Tire Amber Ale logo.
Yes, the south rock is the true summit and a more fun climb, especially at night. Can keep it at class 3. The middle rock is class 4 but usually descended with a rappel, it's bolted at the top.
As far as up the canyon and altitude, I think something like Diamond Peaks from Cameron Pass would get good elevation (11k) in a short hike with views, but is a > 90 min drive each way (the RT driving time would be longer than the hiking time, in TX this is normal). Depends on their beer schedule. Might be better to end the trip with a beer day instead of starting. As far as acclimation, when I visited from TX the first time, I did Flattop and Hallett as an 'acclimation hike'. Just had been doing a bunch of cardio in the gym leading up to the trip so it was fine, didn't plan on going to the summit when I started, it just worked out. Twin Sisters is even easier than that.