This can be any item that we feel the ghost or spirit may wish to interact with.
It may be better to do some earlier research to try and understand what or who you communcating with and making the trigger object relevant to it.
Toys and dolls are good for children, personal artifacts may be good triggers as well. Basically, any small easily moved object that could be relevent to the haunting could classed as a trigger object.
You can get some quite sophisticated trigger objects now with sensors inside to detect motion or EMF's.
"Cat balls" are an example of this. They detect movement and flash an internal light if this is detected.
You can get teddy bears with REM pods inside (see REM pod article).