It is about finding the right balance between formal and casual and making it work together. You have two options.
Playing it safe
A safe way to achieve the smart casual look is to wear a pair of slim fitting jeans or chinos and pair it with a plain coloured t-shirt or dress shirt. Throw on a fitted woolen blazer to complete look. A tie or bow tie is always a classy edition if you wish to dress it up.
What works well with almost every other colour is a pair of navy jeans. Match this with a plain knitted sweater -preferably something with a little colour.
A warm fitted overcoat is a wardrobe-must this season and is ideal for those colder nights.