Rissani is small enough to walk in twenty minutes end to end, but the ksour and Sijilmassa are spread over a few kilometres of palm groves on the edge of town. The circuit touristique is signposted and can be driven, cycled, or walked; expect three to four hours on foot if you want to look at anything.
Guides and taxis. Petit taxis loiter around the souk gates and will run you out to the circuit for a negotiated fare (around 100 dirhams for a two-hour loop). A local guide — easily arranged at any guesthouse — is genuinely worth it: the buildings are not signposted, the context is half the experience, and a guide will get you into ksour you would not otherwise visit.
Bike. A handful of guesthouses rent bicycles, which is the most pleasant way to do the circuit in cool weather.
Onward transport. Shared grand taxis run from the souk to Erfoud, Merzouga, and Errachidia. Negotiate the fare before you get in.