Singleplayer Guide
From Cube Wiki
until then look at Beginners Guide#introSP or try the bots by quin, to play some easy DM offline.
You can try finding some SinglePlayer maps (for Sauerbraten and others) in this listing.
The Singleplayer mode of Cube2 (Sauerbraten) has been even put up for a discussion about No More SP? .. but (for now) .. we will still have the SP modes; but further development/enrichment of it's features is not a priority, or even planned.
A lot of development has been going on inside the BloodFrontier project though.