There are many guided tour offerings in or near National Parks. They range from half day adventures to multi-day adventures. You often do not need to make reservations for the half day to one day trips but it is advisable to go ahead and reserve if your plans are set. You never know how busy the park will be and if a short river rafting trip is on your mind for your adventure, you don't want to be unprepared.
When picking a tour operator, do not hesitate to ask about their credentials and emergency certificates. It is always wiser to know ahead of time if an accident occurs; your guide will be prepared.
Don't forget, the cheapest option is usually the Park Ranger Tours offered by the parks themselves. They are educational and always led by knowledgeable Rangers.