Not everyone has an available USB port on their computer, especially if you have a newer laptop that only has a USB-C port. This simple mouse from Jelly Comb has both a USB receiver and Bluetooth so you can connect using either. You can even quickly switch between the two, letting you control your computer with one and another device like a tablet with the other.
Its low-profile design lets it slip easily into your bag but isn't the most comfortable if you prefer a more ergonomic design. Despite appearances, the ball on top is just a scroll wheel, not a trackball. Also, there are no forward and back buttons on the side if that's a must-have for you. However, a button in the middle of the body on top quickly switches the DPI to give you greater cursor control on the fly.