RMNP for sure! Unless we get a freak May snowstorm, getting to Estes Park will be cake. Don't count on Trail Ridge Rd being open that early though, but it will all depend on how much snow the high country gets this winter. With it being an El Nino year, can probably bet on a few big dumps in March/April. But, it's not to say there are not great spots.
1. RMNP, there is plenty of driving throughout the park from the east side, even if Trial Ridge is not yet open. You can hike up Old Fall River Rd (generally not open till june) in RMNP. It's about 12 miles and 4,000ft of elevation gain, but some beautiful areas, especially just above treeline, but below the Alpine visitor center (which won't be open if Trail Ridge is not). Other countless hikes in RMNP and Indian Peaks Wilderness. If interested in more info, PM me.
2. Red Rocks Park/Amphitheater outside Denver in Morrison. Just off C-470 and Roxborough State Park south of C-470 in Roxborough Park. For Roxborough State Park, sunrise is best as the foothills shield the rock features at sunset.
3. I-70 corridor: Stop in Idaho Springs, quaint mountain town, eclectic little shops. Get off I-70 just before the tunnel and go over Loveland Pass (stop to take your pic with the sign and the 20ft snowbanks that have been snowblown by CO DOT)) and go toward Keystone/Dillon. Lunch at "Backcountry Brewery" in downtown Frisco. Tasty micro brews!
4. If you go to Arches, do yourself a favor and make the drive through Colorado National Monument outside of Grand Junction/Fruita. You won't regret it! And BTW, from the road thru CO Nat. Mon, if you have an SUV, you can gain access to a hiking trail (via a 5 mile lght/mod 4wd road) to the largest concentration of natural arches outside of Arches NP. Any stock 4wd SUV (with low range ideally if it's wet) with decent clearance can make the drive to the trailhead.
5. Garden of the Gods in Colorado Springs.
6. If you don't mind solitude and a dose of the plains...Pawnee Buttes outside of Raymer, CO. Beautiful at sunrise or sunset!
7. A sunrise against the Flatirons. Set up off hwy 93 south of Boulder near El Dorado Canyon. Return to the same spot for a sunset, especially if there are high and mid level clouds.
8. Mid-May is usually the Erie Town Fair. They have a balloon festival which is one of the largest hot air balloon launches outside of the balloon festival in New Mexico. By early next year they should have dates, just google erie town fair colorado. The hot air balloon launch is usually done about 6:30am Sat/Sun mornings at Colorado Nat'l Golf Club (formerly Vista Ridge GC) weather/wind conditions permitting of course. They've only scrubbed 1 launch b/c of winds over 10mph in the last 5 years. The golf course is in Erie off hwy 7 just west of I-25. Free roadside parking, and a suggestion, get there early and go to the volunteer table as many of the crews solicit for volunteer passengers to go up in the balloons for FREE as balast, but it's the early bird that get's the ride, so get there early and you may get a free hot air balloon ride with an incredible view of the mountains to the west. Not exactly a photo op everybody gets. My wife and I and our neigbors go every year as it's less than 10 minutes away.