Hi @Cath62. Do make contact with Dementia Australia for support - they can provide a wealth of information re the 100+ dementias and supports available. They will also point you in the direction of groups, day centres etc in their area.
My hubby likes word puzzles (those find a word pages can keep him quiet for hours) and he does stretch his mind trying to think of words in the 9 letter word puzzles (the basic ones). Perhaps your Mum can be involved in simple crafts ? The day centre will give your Dad a break and give her outside stimulation, too. (My hubby did not want to attend one - he may in the future). All the best to you all.
