The distance, honestly
Chiang Rai is roughly 3 to 3.5 hours from Chiang Mai by road, which means a single-day round trip involves 6 to 7 hours of driving alone, on top of however long you spend at each stop. It's a genuinely long day, and worth going into with realistic expectations rather than assuming it's a quick hop.
What you'd see
Organised day tours typically cover Wat Rong Khun (the "White Temple"), a strikingly modern, all-white temple built by a living Thai artist and covered in intricate, sometimes unconventional imagery; Baan Dam ("Black House"), a dark, artistic museum-residence with a very different, more unsettling aesthetic; and often the Blue Temple (Wat Rong Suea Ten) as well. Some tours add a Golden Triangle stop, near where Thailand, Laos and Myanmar meet.
Day trip vs staying overnight
A day trip is realistic if Chiang Rai's main sights are your only priority and your schedule genuinely doesn't allow more time — you'll see the key stops, just on a tight, tour-bus schedule with limited time at each. Staying one or two nights in Chiang Rai instead lets you see the same sights at a more relaxed pace, avoid the long return drive on the same day, and explore Chiang Rai's own town and countryside beyond the headline attractions.
How to get there
Organised day tours are the most common way to do this as a single day, handling transport and entry logistics together. Flying is also an option if you're doing an overnight stay instead — there are direct flights between Chiang Mai and Chiang Rai, considerably faster than the road trip, though the drive itself passes through scenic countryside if you have the time to enjoy it.
If you can only spare one day, an organised tour is far more efficient than self-driving — it removes the return-drive fatigue from your own itinerary and someone else handles the timing between stops.