I did one last week, kind of made it up so I'm a bit vague but I'll try and remember
Stewing steak - I coated it in flour and browned in olive oil first (I could do this bit in my crock pot, which is best if your slow cooker can do this, otherwise do it in another pan and pour all the oil and scrapings off the bottom of the pan for extra flavour)
Shallots peeled and halved
A couple of sticks of celery finely chopped
Carrots I had some of those little ones as they were on offer so I kept them whole
Clove of garlic chopped up
Parsnips roughly chopped
A good glug of wine (nearly half a bottle )
tin of chopped tomatoes
a couple of splashes of Worcester sauce
a bay leaf
some seasoning
I put the whole lot in together on low for about 8 hours. Was fab!
To be honest, you can put anything you've got hanging around in the fridge, mushrooms would be good, swede, sweet potato... anything