Phu Chi Fa
In an area of outstanding natural beauty, this peak is (was?) a word-of-mouth secret with minimal crowds or touristic tackiness. It is not one of the largest peaks in Northern Thailand but its remote location, winding approach roads and final spectacular view makes it a trek worth making.
The area is today part of Thailand but the area is traditionally a 'Tai-Lao' homeland. The border division between Thailand and Laos created by the Mekong River (visible for the eagle eyed from the summit) is relatively new.
An outstanding array of rolling hills, verdant valleys and elegant peaks.
Flora and fauna
Farmland and orchards/vineyards turn to bush, scrub and grass near the summit.
Chilly, for Thailand, in the morning and a very pleasantly cool temperature throughout the day.
Driving is evidently the easiest option to get to the area, but be aware of the challenging nature of both the road and the (pickup truck) drivers upon it. Cars are available for rent starting at around 1000 baht per day (plus refundable deposit around 20,000 baht) at Chiang Rai airport and a full tank of petrol (~1000 baht) will get you there and back safely.
Directions from Chiang Rai (2.5 to 3 hours)
Leave the city southbound on Route 1 (Phahonyothin Road). Take the turn off for Road 1020 which will lead you south and then east. 1020 eventually splits in a rural town - with a left turnoff continuing as the 1020 and 'straight on' becoming the 1021. Follow the 1021 - you will begin to see signs for Phu Chi Fa and will take a left turnoff for the 1155 before reaching the town of Ngao. This road passes through some incredible scenery but you will need to keep an eye on signs at junctions - eventually you will come to a checkpoint, keep right. A little further you will reach a T junction - take a right, following the signs for Phu Chi Fa Forest Park. From now it is a direct road and you will see signs for the amenities - parking, camping, information, and the 'viewpoint', i.e. the peak.
To get there from Chiang Mai or Chiang Rai, try Thailand Hilltribe Holidays  who offer guide/driver/car service there
Once you're there, it's on foot all the way at these elevations.
Walk the final 750 metres on a dirt path to the summit and be rewarded with the immense beauty of the Thai-Laos border.
Enjoy a traditional song from local children in hilltribe costume as you climb. They will spot you, and those not astonished by your presence (assuming you are not Thai!) start singing - a smile or 'sawaadee kaaa(p)' as you pass by is fine but this is a rare circumstance when rewarding them with a few baht doesn't seem unethical - they are fun and friendly, and fascinated by 'farangs'. Rather than hassling you as they might at Angkor Wat they will return to happily playing shortly and rather than some cabal of gangsters, the money goes straight to their proud parents, invariably a few feet away. Those unwilling to support the 'use' of children for income could instead teach them a little of your language or compliment their song (dee dee!) or attire (suay maahk!).
There is a minimal amount of warm clothing and souveniers available for purchase at the final car park near the summit. Don't pay more than 80 baht for a scarf.
If you're not into instant rice porridge (sold at stalls near the peak), pack a lunch. There is claimed to be a 'pub/restaurant' near the top but we could not find it.
There are homestays available nearby. Keep an eye out for signage.
There is camping available very close to the summit. Essential for sunrise viewing.
You will pass some rather affluent looking townlands on the way up the mountains.
Don't push your luck with the edges of the climb, and drive cautiously. If tired, pull over somewhere safe and take a nap. Alertness is needed on these crazy corners which the locals have no worries about overtaking on.
There is little choice but to retrace your steps for most of the journey. Taking the 1021 eastward rather than westward back to the 1020 will bring you through Phayao Province, near Phayao itself and to Chiang Mai along very obscure roads in a little under four hours.