I often have left over mash. The next day I reheat it in the microwave, mix in a little bit of butter and lots of grated cheese, then either eat it as it is or serve with baked beans, Mmm yummy!
if you like fish make fishcakes, smoked haddock is great with fresh parsley and mash, or rissoles, left over meat or corned beef, onions preferably partly cooked,as the other peep said bubble and squeak fantastic, you apparently can use left over mash to make a chocolate cake, umm hungry happy eating