Even though he may be a monk, Miroku doesn't act like it all the time. He takes advantage of the wealth of various rich families on his travels and he's a horrible letch. However, when you get past his faults, he's a true monk in terms of his wisdom and spiritual powers. In fact, many will agree that he's the second most powerful character in turms of spiritual abilities, second only to Kikyou herself.
Among Miroku's abilities, aside from the Kazana of course, is his ability with his shakujou. A shakujou is what we would call a typical Buddhist staff. Other famous priests and monks in anime (ie: Rezo from Slayers, Chichiri from Fushigi Yuugi) carry these staffs around so it's pretty common. Miroku's abilities with his own shakujou are very skilled. He can easily knock thrown objects out of the air with it and he even succeeded in cutting the arm off of one of Naraku's puppets! Miroku's shakujou is a very effective weapon, wether he's bonking someone over the head with it or throwing it to knock an opponent down out of the air. Aside from the kazana, the shakujou is Miroku's primary weapon.
Miroku can also use special pieces of paper called ofuda. Again, many characters in anime have used ofuda (ie: Hino Rei from Sailormoon, Tasuki from Fushigi Yuugi, Shaoran from Cardcaptor Sakura) and ofuda actually exist in Japan. Miroku generally uses his ofuda as weapons against youkai and also places them on walls and doors of various houses to ward off evil spirits. They act as excellent long-range weapons against youkai and considering Miroku's great spiritual power, they're very strong. They also happen to come in handy when facing illusions created by youkai.
Spiritual salt is also used occasionally by Miroku. When the salt comes in contact with a youkai or spirit, it causes damage to them. Miroku doesn't seem to use this very often but it seems to be rather effective. In the Inuyasha fighting game for the Playstation One, one of Miroku's moves is throwing salt at his opponent. ^^;
Miroku has endured hard training, as evident by his knowledge and skills. His training has allowed him to be exposed to large amounts of shouki (evil aura) that would kill a normal human being. Of course, this doesn't mean that he doesn't feel somewhat sick when in environments where there is a lot of evil but he can hold out for a while. This comes in very handy since there are a number of occasions where Inuyasha's group is forced to be in large amounts of this evil. The more in the group that can stand up to such large amounts of evil, the better.
On a number of different occasions, we have seen Miroku erect a barrier. Once was with Kaede's help when trying to hide a wounded Inuyasha and another time was when he was hiding his own self from a hoarde of youkai wanting to kill him. Miroku can keep a barrier up for some time but it takes concentration and the more complicated and larger a barrier is, the more concentration it takes to keep it up. In the case where he and Kaede erected a barrier to shield youkai from other youkai, Miroku was unable to move from his position. Miroku can also keep a barrier up for a fair amount of time while injured or nearly unconcious. This just goes to show how powerful he really is.
Lastly, Miroku's status as a monk gives him some intuition. He's able to see through various trickery brought forth by youkai and he knows various ways of expelling youkai and getting rid of them. He may not be quite as good as Sango when it comes to figuring out the most effective way to kill a youkai but his abilities are just as good, if not better than, Sango's.