What size pack do I need?
A consideration when making this choice is in deciding what types of activities are you going participate in the most. If you see yourself mostly sticking to road cycling an hour or two, the smaller and simpler, the better. Pack's like the CamelBak Classic or our Top Pick for a Lightweight Pack CamelBak Rogue should serve your needs with their focus on carrying just the essentials like a spare tube, tire levers, and CO2 cartridges.
Different Types:
Though hydration specific packs are used commonly in some sports from climbing to mountain biking, certain designs are best suited for certain sports, though many of them overlap. Many of the full-sized packs are multi-purpose and will be suitable for many sports. However, if you are looking specifically for a running pack or a mountain biking pack, you should look for certain features that would be beneficial to you. For example, some mountain bike packs have been integrated with spine protectors, and most sports helmet carrying systems for long gravel climbs.
Lightweight:
These models we see as nothing but water carriers. Though they might have some small pockets for keys and a cell phone, they are not built to be a multi-purpose daypack that also offers hydration. Despite their diminutive size, these packs are typically very stable and comfortable, and suitable for trail running.