I have a love-hate relationship with my freezer because I often forget what is in it. Every time I put leftovers in it, they are still there 6 months later! This is why I don't tend to batch cook.
However, there are some staples that I keep in there. I haven't included the obvious ones such as bread, milk & meat.
Mixed casserole vegetables
Dairy free cream
Herbs- if you have fresh herbs that are near to expiry, put them in the freezer & use them when you need them
Cut up banana that is too ripe to eat but perfect for smoothies
Little bags with leftover red wine/white wine ( a brilliant trick learnt from Nigella Lawson. Perfect for when you just want to add a glass to bolognese/stew/casserole)
Breadcrumbs-any time you have stale bread, don't throw it away, blitz it and store in the freezer
Home made crumble mixture- make a double batch of topping next time you make a crumble, so that you can always make a quick dessert