You want to mix it up, do more than one thing. Throw in your weeds when you are gardening (especially if it has a big hunk of soil attached, there are bugs in there for them), toss in veggie scraps from dinner (like bell pepper seeds, cob from corn on the cob, water melon rinds ect) , scatter some meal worms, put in an old chair or other stuff from around the house the can stand on and go under, hang treats like folks said, if they don't like cabbage try something else. Sprout some seeds for them. Add herbs that are good for them to their regular pellets for a change in flavors-like oregano, cayenne, cinnamon ect. Grow some container plants and set the whole container in there-easy things can be radishes, pansies, lettuce, alfalfa ect. If you can afford it get a safe container shrub, or small tree and plant it in there for them to sit under, destroy, eat ect.
Some things will be bigger hits than others with your group so just start trying things to see what they like.
Have fun with it.