Manali is busy, and the trick is knowing where it is not. We stay in Old Manali rather than the Mall Road crush, start the Solang and Atal Tunnel day early enough to beat the queue at the tunnel mouth, and keep an afternoon free for the Hadimba deodar grove when the tour buses have gone.
The Atal Tunnel run takes you through to Sissu on the Lahaul side — a completely different landscape, bare and enormous, forty minutes from a green valley. In winter this is the trip's highlight; in summer it is the coolest half-day you will have.
Nothing here is a route march. Two sightseeing days, one river day, and enough unscheduled time that the trip belongs to you rather than to a clipboard.