Hi,
I'm planning a trip to Yunnan in June 2018 for about 3 weeks with a few friends. We are looking for places to visit that are not completely filled with tourists but still worth visiting. Old towns, hiking, beautiful scenery, tea houses, perhaps swimming, are all of interest too us. I speak decent Mandarin which I imagine should help us get around.

In addition to specific places, it would also be great to get some general tips/advice for how to travel "off the beaten path" in Yunnan.

(I know this has been asked before, but I figured I would ask again to get more recent information.)

Depart Tiger Leaping Gorge from the east ring road. 1000s enter from the front. Only a handful go out the back.

General off beaten advice...If the horde of Chinese tourists go left, you go right. Enter any major site an hour or two after opening time.

I would agree with AlexKMG that Tiger Leaping Gorge is simply a must:

www.gokunming.com/[...]
As for other ideas, check out our article on Yunnanyi:

www.gokunming.com/[...]

Maybe click the tag 'old towns' for more ideas. Happy traveling!

Sorry Stevula but you are at least five years to late. No more unspoiled off the beaten track old towns. Places like Bamei, Heshun, Nuodeng, Donglianhua, Weishan, Banqiaozhen, Shaxi and Jianshui are all taken over by mass tourism like Dali and Lijiang and Shangrila. That is droves of package tour tourist being pushed passed tourist crab dealers back to the especially build parking places and visitor centres. There are no more teahouses except those selling overpriced Puer tea.
A lot of the special landscapes now have viewing platform (Hani terraces, red earth, Renzi Bridge) or are turned into tourist parks (Stone Forest, Sand Forest). Even historical sites like the Songshan Battlefield have been turned into theme parks.
But Yunnan still has wonderful scenic area but then indeed you have to do as suggested above; if everybody goes left, you go right and vice versa. Timing things works as well; wait till the tour busses are gone or have gone for lunch.
You still can have a great time if you go to those places nobody else goes and just absorb the atmosphere and who knows you will find a little gem.
Hiking is still great in the Nujiang Valley. The Haba area just north of the Tiger Leaping Gorge is great for hiking as well.
Another tip: Don’t visit star rated tourist attractions; stars mean overpriced and overcrowded rip off of little interest. A five star rating means “Avoid at all cost”.
Sorry but Yunnan is not a swimmers place. The lakes are polluted and the rivers to dangerous (Cold). I only swim at some hot spring places (A few metres).

If you really like hiking you can follow the old Yunnan - Vietnam railroad. It goes through some of the most remote and beautiful areas of Yunnan.

Thanks so much everyone so far. This is all very helpful and exactly the type of advice I'm looking for. Keep it coming.

That's very sad to hear that all the old towns are completely overrun by tourists. I imagine some of them must still be at least worth visiting, if only for a short period? If so, what are a few that would best? Perhaps Yunnanyi, as GoKunming suggested?

I think we do plan to visit TLG as a few of you have suggested, despite the crowds. Yunnan-Vietnam railroad is an interesting idea, I'll look into that.

I'm also wondering about transportation to/around less traveled areas. Can we rely on public transport to get us around or is it too sparse? I suppose we could hitchhike or hire a car? Or we could rent scooters and do day trips from a town?

Do include Heijing on your travels.
www.gokunming.com/[...]

None of the places above are completely filled with tourists off peak time. So the first 3 weeks of June you will be fine. Peak time don’t even attempt to bother.

Heijing is a special case. Because it is in easy driving distance from Kunming the weekends see the town filled to the brim by Kunmingneese and their SUV’s. Prices triple and everything is full. During the week you will have the whole place to yourself. Question of timing. But during the week the place has a bit of a Brighton in winter atmosphere.

Yunnanyi is a special case as well. Several times they tried to market the place as a tourist destination. They widened the central road by cutting about two metres off the houses lining it. They created two museums: One related to the Flying Tigers and the role of the local airfield during WWII. The second related to the function the town had as a staging point on the teahorse route. The first museum has undergone a complete rebuild but never opened again (And frankly was a crab museum). The teahorse place was trashed during filming of a TV series (Yunnanyi is often used as a base for filming and then the whole old town is closed. For years replica war planes, used in a film, were standing at the parking place but they have all rotten away.) and never again opened. (Information based on my last trip past there in October).
They are recreating the place again as a tourist attraction; that is they build a new visitor centre again and build a new toilet block. Both were locked up last time I was there. There is a hideous new old gate build in the middle of town.
Still there are some little gems all only possible because of their failure to make it a tourist town.
At the south side of the main street next to the kindergarten an old man created his own small Flying Tiger Museum, you are lucky if he is there and you can get in. Some folks make straw hats as used by the local farmers and are willing to show you how it is done.
Up the hill behind the village is the main temple. If you are lucky it is a special day and you will groups of people preparing food on one of the many public cooking places at the back. I got several times invited to join them.

there are still 100s of unexplored off the beaten track old towns in yunnan. a clue: take buses, not trains