If you don't have a convertible rucksack with a detachable daysack, you will need to combine your rucksack with a smaller carrying vessel. A shoulder bag works very well in conjunction with a a rucksack or daypack on your back.
Waist bags are a very comfortable way to carry your bits and pieces offering easy access and certain amount of added security. Some have water bottle compartments - ideal for walking/cycling. They work well with a day pack to keep your camera in or wet wipes or any other bits and pieces you may need in a hurry.
It gives easy access and can be used on its own when you can lighten the load. Make sure it can take the essentials - water bottle, guide book, phone, camera.