For each camera, you must have at least one extra battery (or set of batteries) and an extra memory card. Do not forget to buy a suitable bag, especially if you have a DSLR-camera. This bag should be lager then you really need at the time.
Apart from the extra battery and memory card you will in the course of time "collect" all different stuff. Think of a flash, a filter, a cloth to clean your lens to make an extra lens, etc. Bit of a shame if you have to buy a larger bag every year.
Something that obviously does not fit in your bag is a tripod. A “must have” if you want to deal seriously with photography, especially in lesser weather can stand an outcome.