Best easiest day trip
Idaho Springs because it preserves the day instead of consuming it.
Day-trip feeder
Best day trip from Denver into the mountains
Idaho Springs is the best easy day trip. Estes Park is the better scenic day trip. If a longer drive is the only way the destination works, it is probably no longer the right day trip.
Best scenic day trip
Estes Park if visual payoff matters more than the simplest logistics.
Hard rule
If the drive pushes into full-day fatigue, the answer is usually wrong for this page.
Short answer