Lahul and Spiti’s trails, though well frequented in high season, are long, hard and high, so you must be self-sufficient and have a guide. Pack-horses and provisions are most readily available in Manali, or in Keylong and Darcha (Lahaul) and Kaza (Spiti) if you can afford to wait a few days. A good rope for river crossings will be useful, particularly in summer when the water levels are at their highest.
The best time to trek is July to early September, when brilliant blue skies make this an ideal alternative to the monsoon-prone Kullu Valley. By late September, the risk of snowfall deters many visitors from the longer expeditions. Whenever you leave, allow enough time to acclimatize to the altitude before attempting any big passes: AMS (Acute Mountain Sickness) claims victims here every seasonT.
Lahaul: Darcha to Padum via the Shingo La Pass
The most popular trek is from Darcha over the Shingo La pass (5000m) to Padum in Zanskar. The trail passes through Kargyak, the highest village in Zanskar, and follows the Kargyak Valley down to its confluence with the Tsarap at Purne. There is a small café, shop, and camping ground here and it’s a good base for the side trip to Phuktal gompa, one of the most spectacular sights in Zanskar. During the high season (July & Aug), a string of chai stall/tent camps spring up at intervals along the well-worn trail through the Tsarap Valley to Padum, meaning that you can manage without a guide or ponies from here on. Do not bank on finding food and shelter here at the start or end of the season.
Lahaul: Batal to Baralacha Pass
Lahaul’s other popular trekking route follows the River Chandra north to its source at the Baralacha Pass (4920m) and makes a good extension to the Hampta Pass hike. Alternatively, catch a Kaza bus from Manali to the trailhead at Batal (3960m) below the Kunzum La (4551m). The beautiful milky-blue Chandratal (“Moon”) Lake is a relentless ascent of 7hr from Batal, with stunning views south across the world’s longest glacier, Bara Shigri, and the forbidding north face of the White Sail massif (6451m). The next campground is at Tokping Yongma torrent. Tokpo Yongma, several hours further up, is the second of the two big side-torrents and is much easier to ford early in the morning; from here it is a steady climb up to the Baralacha Pass. You can then continue to Zanskar via the Phirtse La, or pick up transport (prearranged if possible) down to Keylong and Manali or onwards to Leh.
Spiti: Kaza via the Pin Valley to Manikaran or Wangtu
One of the best treks in Spiti is up the Pin Valley. The track alongside the River Pin, which passes a string of traditional settlements and monasteries, is now motorable as far as Mudh, around 40km south of Kaza. Over the next few years it is expected to be paved right through to Wangtu, but for now it forks beyond Mudh into two walking paths: the northern path over the Pin–Parvati Pass (5400m) to Manikaran in the Parvati Valley, and the southern one to Wangtu in Kinnaur via the Bhaba Pass (4865m). The last section to Wangtu itself has also fallen to the roadbuilders, so you might decide just to hitch a ride.