A fresh coat of paint is always the biggest bang for your buck. Most cabinet surfaces can be repainted with a high quality primer and specialty paint.
If you’re happy with the overall layout of your kitchen but have outdated or damaged doors, it’s worth looking into refacing your cabinets
If replacing your doors/drawer fronts completely is not an option, consider updating them with molding.
In our own kitchen, we added individual feet to the bottom of the cabinets, which is the simplest way to dress them up