easy on oak leaves due to high tannin levels, no acorns. they love hawthorn - BUT the spikes can lead to really bad abscesses in feet, mouth, cheeks and if fed in shed the branches are difficult to pick up again without gloves. So I don't feed them to mine.
Hazel, any kind of willow, some ash and sycamore (both lead to looser droppings, so only in moderation), willowherb, raspberries branches, any kind of currant, brambles (again gloves needed), some kinds of thistles (cut off not rooted out - soil on root has unwelcome bacteria in it), nettles - wilted or not, see what yours prefer (gloves and long sleeved jumpers essential), mine also love comfrey and horseradish leaves, any kind of cabbage peelings (like the inner stalk of broccoli - always cut outside my kitchen of course
), pony carrots (cut into long sticks), apple/pear slices, bananas (peeled and skin fed separately), I grow a few beds of kale (for both goats and humans), dock leaves (I specialise on huge dock leaves on the edge of my polytunnel)... mmhhh I am sure there are others....