On this page you'll find a selection of day backpacks- those that are intended for single day hikes and outdoor activities. They can also be used for school or college, or for days out around town. In general, they range between about 20 and 35 liters in capacity. 20L packs are on the small side for a full day hike but you should still be able to fit a waterproof jacket and your lunch in there, no problem. If you're going a little further or will need to carry extra clothing in winter, look for something around the 30L mark
30L packs should give the average summer hiker plenty of room for food, water, and spare clothing, and they're usually big enough for basic winter hiking too. Look for features like an adjustable carry system, a padded waist belt, and space for a hydration bag if you're going to be tackling long routes or hiking all day. The basic design of the pack is also worth considering- top loading bags are tougher and more durable when it comes to carrying heavy weights but a front-loading style makes it easier to find small items quickly. Whatever the design, a few internal or external pockets often come in handy. Look for one that would suit your cell phone, for instance.