Milton Keynes

Here's what I carry on my bike:

Water bottle in cage, necessary during the hot summer
Underseat bag with extra tube, patch kit, tire levers, Shrader-to-presta adaptor, multi-tool (saved me once on my mountain bike, 10 miles from home with loose left pedal), ziploc bags (in case of rain I can put my wallet and phone in them), and a few zip ties for temporary repairs, just in case.
Air pump fastened to side of seat tube, has fittings for both Schrader and presta
"Glove box" small pack just behind handlebars on the top tube used to hold my keys, wallet, pocketknife, etc.
Cell phone on handlebar phone mount

Looking into getting a small first aid kit like the one I have in my fishing tacklebox. Might just use that one since I've never had to use it while fishing. Has band-aids, antibacterial cream, small pieces of gauze, etc. Might throw in a few folded up paper towels for "just in case." Is inside a plastic box about the size of a bar of soap, will put it in the under seat bag.

elizwlsn

I have a larger handlebar bag with a front pocket. I carry in the front pocket a spare tire, levers, pump, a few small tools, a ziplock with a rag and a couple of latex gloves that have come in handy more than once. Inside the main bag I carry tissues, aspirin in a small tin, a clean facecloth in a ziplock to wet to cool down when it's hot, sometimes an extra water bottle, my phone and ear phones, a small coin purse with my license, a credit card, health insurance card and some cash. I also keep a small notebook and pen in there. I am out in the country and usually go for 20 - 30 mile rides so want to be prepared. I also keep two 24 oz bottles that I half fill and keep in the freezer, then finish filling up before I leave.

Little Darwin

I always ride with a trunk bag on my rear rack.

For flats: Tire levers, at least 1 spare tube, a patch kit, CO2 inflater with at least 2 cartridges, topeak road morph pump, schreader adapter for my presta valves... I hate the idea of pushing my bike because of a flat. Ironically, I have never had to fix my own flat while riding, but have helped others.

Other stuff: Multi-tool, rain poncho, a couple of bungie cords, first aid kit, space blanket.

Food/hydration varies, but I usually carry 2 24 ounce water bottles on the frame, and usually some fruit/nuts/snack food. On a hot day, I will often carry a third water bottle.

My rides tend to be on isolated rail trails, so I hope for the best, and prepare for the worst. Since a main reason I ride is fitness, I don't mind the weight... plus, I weigh more than most tandem teams, so definitely not a big deal to carry an extra pound or two of stuff.

Calla lily

In a small handlebar bag I carry mini pump, spare tube, patch kit, tire levers, multi-tool, small rag, phone, keys, chapstick, a couple hair elastics (for my hair and also to hold a wahoo rflkt on the stem since bands that came with the mount kept breaking), id and credit card/cash if I remember, sometimes a small snack if a longer ride. And that's all that will fit in the bag. 1 water bottle in cage. I only have 1 cage mount, so hot days or longer rides I also wear a camelbak. I'll probably get a rear rack and bag eventually to carry the above and extra water (so I don't have to wear the camelbak) and whatever else I want to bring on longer rides.

Milton Keynes

OK, I pulled that first aid kit out of my tackle box and found that the plastic box was a bit too big for my under seat bag with all the other stuff I've got in there. So I pulled all the bandaids, gauze, and antibacterial cream and put it into a small, snack-sized ziploc along with a couple of folded-up paper towels. I figure if I take a spill and skin up an elbow or knee, I can wipe it down with one of the paper towels and a bit of clean water from my water bottle until I can get some gauze on it. And you never know when paper towels might come in handy. I may throw a few more into another ziploc bag and store them in the under seat pack. There's been times while riding with my son he needed to wipe his nose, and I didn't have anything with me. I guess that's what shirt sleeves are for.