I would hold off on any med ball throwing until your PT evaluates your ability to maintain neutral with activity in person.
As for core stuff to do ... there is plenty ... planks and plank versions are probably the best. Add in something like elbow touches, push ups, and chin ups and you are working the abs pretty fully. But again, it depends upon what position your spine is in when you are doing these things. Your PT will be able to direct this better after he/she evaluates you in person.
Robin McKenzie's book explains it best, but
here is a decent article on maintaining neutral.