Southeast Asia forum

Well, you can overland from Northern Laos into China fairly easily if you setup your Visa into China before you get to the boarder. 3 weeks is ample time to get to Kunming - there isn't that much to see between Laos and Kunming, a stop in Jinhong will lead you onto a few side trips around xishuangbanna, not necessarily as nice as you'll get in Northern Laos though. If you cross in at Huay Xia maybe take the boat down to Luang Prabang (a not to miss). From there, with the time you have, you could go out to the Jar of Plains and loop back the long way via Sam Nuea and around to Nong Kiaow/Mung Ngoi. From there head up to Luang Nam Tha and Mung Sing and you'll be able move back to the border at Mengla.

If you like you could probably take a trip down to Vang Vieng and Vientiane then double back to the north as well if you don't like the idea of heading out to Phonsovan. A few years back I took a trip from Kunming and did a loop as the one I outlined above (minus Luang Nam Tha) and it took roughly 3 weeks. Perhaps save some time on your arrival to Kunming to check out the Stone Forest, I haven't, but it looks pretty cool.