If you expect to run into different temperatures, wear a t-shirt under your shirt, and leave the shirt open. When it gets hot, just take your shirt off, and you’ll be comfortable in your t-shirt. When it gets cooler again, simply put your shirt back on, and it will act as a jacket.
If the day's temperatures are going to be milder, you’ll be fine with just a shirt and a jacket. Create your basic autumn look as a fall-back for those times when you don't know what to wear. You can't go wrong, and it will save you some time and stress each day!
For a practical, uncomplicated solution, but one with personality, go for looks with jeans for work, and then add a shirt. Plain, striped or checked, choose the shirt that reflects your personality, and feel good in your own skin.