miniature for epic was weilding three swords...
Train. Then train some more. And maybe a bit more training.
Though it depends on the Chapter. Space Marine, though only semi-canon, is probably the only novel that really deals with (non-Wolf) Marine downtime. The Imperial Fists pray, fight honour duels, scrimshaw bones, study in the library and even seem to just 'hang around' at times.
Salamanders go amongst their local populace and train at the anvil.
Blood Angels do artificing type stuff - creating uber buff chestplates and such like.
Space Wolves party, tell tall tales and stay up late braiding each others'
hair and talking about which Chapter Master is the hunkiest (Azreal's so dark and brooding! No, Calgar's way more buff! Well I think Dante's just hte cutest!)beards.
Iron Hands presumably do tech related things for amusement.
Ultramarines stand in front of the mirror and flex while telling themselves how awesome they are and that everyone else is just jealous.
Dark Angels brood and skulk in the shadows.
Lamenters write terrible, depressing poetry.
(some of the last ones may not be canon).
Also, exclusive Black Library limited edition about Chaplain Cassius - Catachisms of Hate. Poor Gav, having to number and sign 1,500 copies. I'd reckon they'd be pretty wobbly signatures towards the end of it.