I use my hip pockets mostly for holding snacks or other items that I want regular and immediate access to .
When I go out on a hike I pack both hip pockets full of granola bars and other goodies, then I don't have to stop on the trail when I want a quick snack.
I keep my keys in the hip pockets on the small mountain biking bag
I carry every day, as well as a couple tools for making small adjustments on my bike while I'm riding.
I'll also keep one or two food bars in there just in case of a snack attack.
Other things you can keep in there can include: pocket knife compass small headlamp/flashlight extra batteries small point and shoot camera chapstick hand sanitizer pretty much any item you would normally keep in your pants pockets, but don't want to pack them full while hiking.
You're
needs may vary depending on your activity, but the intent of the hip pockets is to keep the small items that you are going to want to be able to have quick access to without stopping and taking off your backpack.
outdoors.stackexchange.com/users/4148/ShemSeger
It depends on the size of the pocket, so here are a few very different ideas for small things you might want to get at without stopping (for long): Many I've seen would take a small phone or compact camera.
I've used them to support the lens tripod mount when hiking with a SLR and long zoom lens, to reuce the load on my neck.
Similarly a GPS (like an etrex) fits in there, though I carry mine on the shoulder strap for better signal and access.
A compass is handy if you don't want to carry it round your neck.
When travel-backpacking (as opposed to wilderness hiking), this can be a good place to keep tickets, passports and money.
Again the phone or source of music if you want one.
When moving fast (speed hiking or trail running)
energy snacks can go in there for access on the go.
Quick access to water would be needed too (Camelbak or bottle in a belt holster).
If you like to whittle, a folding knife can go in there.
Smokers may find them quite handy too (though hopefully not in the wilderness).
One unusual but very useful item: a mirror in the hip pocket of a child-carrying backpack to check if the child is awake.
outdoors.stackexchange.com/users/2529/Chris H
In my 2-3 days outings backpack (45L), I wasn't sure what to put in the hipbelt pocket.
Then I noticed my 0.3 Adventure Medical Kit fits in there quite nicely.
It's really just the basics like bandages, gauze, and plasters but those things can be dealt with a backpack on.
Before that I had a pocket chain-saw in that pocket
but I stopped carrying it.
outdoors.stackexchange.com/users/15955/Gabriel C.
Along with all the other answers, I would add toilet paper (in a waterproof bag obviously).
Nice to keep it handy, when you need to go you don't want to have to unpack your entire bag to find it buried at the bottom.
outdoors.stackexchange.com/users/16430/T_Bacon
Stuff happens when least expected.
You day hike could turn into an overnight stay quickly and unexectedly.
Be prepare for that situation.
Use that pocket to hold an emergency item such as, but not limited to, an extra lighter and/or tinder, space blanket, folding knife, hand sanitizer, water purification tablets, or anything else that would be helpful during an overnight stay.
outdoors.stackexchange.com/users/11800/B540Glenn
Keep condoms in that pocket.
Seriously.
Besides the obvious, a condom is useful if you need to keep your phone, keys (including electronic buttons), or anything else small, dry.
Instructions for use: put small item inside the condom & tie the loose end in a knot.
outdoors.stackexchange.com/users/13785/DaveDev
My hip pocket is wide enough that it can handle a smartphone turned sideways.
I store mine here inside one or more ziplock bags to keep it dry.
Storing it here raises the risk of damaging the device if you fall the wrong way, but overall the convenience is worth it for quick photos or directions.
outdoors.stackexchange.com/users/3497/David Metcalfe
My rucksack has a small pocket on the hip belt, but I can’t think of anything useful to keep in it while day hiking.  What types of things is such a pocket intended for?  If there is no common purpose for such a pocket, what are some things I could keep in it that would be useful?
What could I keep in rucksack hip pocket?
outdoors.stackexchange.com
<hiking><backpacks>
outdoors.stackexchange.com/questions/20533
outdoors.stackexchange.com/users/16176/Andy
This pocket is widely used for a phone, GPS, compass, keys, condoms, a compact camera, snacks, cigarettes, a pocket chainsaw, medical kit, toilet paper,  tickets, passports, money or a folding knife. Toilet paper, condoms, tools, devices, documents, medical kits, snacks and water can be kept in hip pockets.
Toilet paper, condoms, tools, mirrors and cigarettes can go in hip pockets. Snacks, water and medical kits can fit. Phones, cameras, compasses, documents can be kept there.