I suggest looking at the ki-throw feat chain for the monk half, eventually you can throw enemies into others and provoke trips on all of them.
With druid, there are all the various animals that can provoke with trips, but also various spells on the druid's spell list, such as the early contender grease.
There's a really bizarre tripping feat chain for quarterstaffs that leads into tripping twirl and quarterstaff lets you get weapon specialization for quarterstaffs, but its probably too feat and BAB intensive for this build.
If you're DM allows monster feats and you have a high strength, awesome blow allows for a different method that trips and kind of bull rushes at the same time.
Regardless, if your build has enough feats vicious stomp can be quite fun.
