As to what veg to put in, besides the obvious onion and potato, for my multi-veg base in winter I usually keep larger cut-and-come-again root veg in the fridge - swede, celeriac, turnip. Celeriac does go brown and manky-looking once cut, but a thin slice is easily taken off.
Dh prefers non blended soups now*, but often, when we had guests after Christmas I used to make a blended ‘root soup’ for lunch - onion, potato, carrot, and swede, plus of course stock cubes - with milk and a little butter added at the end. And some chopped parsley if I had any. Always went down very well.
*Though having said that, I have a jar of roasted red peppers that’s been sitting in a cupboard for ages - so there’s going to be a tomato and toasted red pepper soup coming up soon.