first probably the easiest is Java moss which looks good on anything rocks or driftwood and it grows in all directions up, downish, sideways, they say it grows slow maybe compared to stem plants but in my opinion it grows relatively fast for a moss.
or you could do christmas which also looks good but it usually grows sideways only good for moss walls though, i think this could be one of the slower growing mosses and is very hardy. i ordered some of this off eBay and didn't know it was coming from Thailand a week and a half later it came and it was all brown except for some small parts so i blasted it with high lights co2 and ferts and it came back like a champ.
Flame moss looks good but may be a little harder and when i say little it means that it barely needs more light but it basically grows in all conditions but it grows straight up which could look really good on driftwood, it grows at a moderate pace, it usually stays 3-4 inches in my experience.
or fissiden fontanus moss it looks particularly good on driftwood, super slow grower, this is a harder to find moss but you could most likely find it on eBay or if you search it on google.
i just learned about this species thanks to Old Scales build thread