Yeah, I imagine it won't in April... but yes, Red Rocks is totally worth a trip.
There's always Estes Park/Rocky Mountain National Park, which is definitely worth the haul but more than an hour... but there are gorgeous places very close to Denver, and if the weather is nice you can just hop on 70 W or 285 S for a bit and see some gorgeous stuff driving.
There's St. Mary's Glacier, which is a short but gorgeous hike pretty close to 70- and also near Idaho Springs, which is a fun little mining town where you can eat/shop and take in some history/scenery. There's also Lookout Mountain off 70.
If you take 285 there isn't a whole lot other than scenery but it's totally beautiful (leads to the gorgeous South Park basin area)... and there's a giant bizarre hot dog-shaped restaurant called Coney Island where you can grab a bite to eat
I don't know the Ft. Collins area very well, other than several of the breweries
... if you're into beer, we can also talk about that