On a hike, everything you want or need will be carried with you. If you can get by with a granola bar and bottle of water, then you've got a light load. That camera, tripod, cellphone, bottle of wine, kitchen sink will all add extra weight right on your back. So, everything you leave at home is weight off your back. Those things you do need to bring should be carried as comfortably and easily as possible. Choosing an adequate, well-fitted pack will make your hikes much more enjoyable.
There are three general sizes of pack you might consider for your hiking trips. Depending on the size of your group, length of your trip, weather, and extra gear you need for activities, you should choose the smallest size required.
Also called waist packs, lumbar packs, or hip packs, these small pouches are intended to carry personal belongings on day hikes. Fanny packs are light and very useful. They have many different features such as built in water bottle holders, compartments for different gear, and loops to hang stuff.
A day hiking pack has shoulder straps but no waist belt. It is intended to carry lighter loads on the shoulders for which you do not need the waist support. There is no sharp line between hiking day packs and back packs - starting with tiny packs, you can find larger and larger and larger packs, some with hydration systems, some with added chest strap, and some with waist strap.
For day hikes, you really shouldn't need a hiking back pack unless you are geared up for cold weather, are going a long distance, or are carrying gear for others. If you are training for an extended trek, then using your backpack whenever you can is a good idea, too.
Whichever kind of pack you decide to use, make sure you are able to keep the contents safe from rain. A fanny pack should fit under your rain coat and you should have a cover for your hiking day pack or back pack.
Get a pack based on its features and fit, not on fashion. No one really looks cool wearing a pack anyway, so don't worry about it. But, try to get neutral, earthy colors to lessen your visual impact on the environment.