The best piece of advice we can give is to make sure it’s big enough! As a minimum you need to fit in your lunch, water bottle/s, wet weather gear, warm jumper, and other personal bits and pieces. A larger pack weighs marginally more, costs very little extra and you’re unlikely to regret it.
Well-padded shoulder straps are a must as your pack is inevitably going to feel heavier as the day goes on. Padded waist/hip belts are also very useful as they help take some of the load off your shoulders. A chest strap is available on most good packs these days and it will stop the pack from sliding around on your back whilst you bend over or walk on uneven terrain.
Several pockets or sections can also be handy, allowing you quick access to things like sunscreen, water bottle or snacks.
Most packs are not waterproof so it’s always a good idea to line the pack with a heavy-duty plastic bag. You can also buy a pack cover, which does an excellent job of keeping most of the rain out.
Some daypacks have a curved back and this allows more air to circulate, making for a much more comfortable experience.