A pair of beige/nude wedge sandals, not too high, they'll go with anything in any colour, jeans, dresses, skirts. Try New Look.
White trainers.
Pretty flip flop type sandals, Primark are cheap and they last, had some of mine for years.
Slider type sandals with one or two straps across the foot, I've just bought some new ones in Primark, beige, mint green, gold (two thin straps with jewels)
I went out for lunch yesterday and wore a light brown top, cream ankle length trousers (H&M) and my new gold sliders, I looked smart / casual.
For other ideas look online at clothes sites, New Look, River Island or whatever shops you buy from then look at the models feet and see what shoes they're wearing with say, casual trousers, smart trousers, skirts, dresses.
I've just bought some smart wide leg trousers, struggled with finding the right shoe so I looked at the model online and she was wearing a flat sandal, perfect.
For the winter, an ankle boot, a knee high boot, a pair of brogues.
The ankle boot for jeans, knee high boot for dresses and skirts, brogues for dresses and skirts with thick tights