Very near Castle Rock is Castlewood Canyon park. It's a real nice walk with lots of interesting views. Your sister will know all about it.
If you head down to Garden of the Gods, try the back way from Castle Rock, down 105. It goes almost in the shadow of the front range, is prettier than I-25 and there might be a few shots worth taking. Also on the way is the Air Force Academy. The chapel is pretty amazing architecture.
A huge, loud second to toro_mike's recommendation of Daniels Park. Sometimes it's hard to find the Bison and sometimes they are right next to the road. But the view from Daniels Ridge is ALWAYS spectacular. I try to stop there for a few minutes on my Sunday trip to the grocery store. It's a great place for a sunset picnic. A few miles north of there is Bluffs Regional Park which is the very northern end of the same high ground. From there you overlook the entire metro with the mountains in the background.
Bluffs Regional Park address is:
10099 Crooked Stick Place
Lone Tree, Colorado 80124
If you will go to Estes Park, definitely check out the Peak to Peak (hwy 72). And stop at Red Rocks while you're at it, probably the most beautiful natural amphitheatre on earth. And if you're a flatlander, don't attempt to run the stairs. The surrounding park has endless examples of Colorado red rock formations.