Saddle Bag
It may seem too small at first.
But check what I can fit in with my Specialized Saddle Bag : Two spare inner tube boxes Cell phone Keys Money Train travel card 3 tire levers Small toolset
bicycles.stackexchange.com/users/131/b.roth
I just picked up a Timbuk2 Handlebar Bag , and so far am loving it.
Has a nice velcro-down pouch, and attached to the bike via two velcro strips.
It is on the bike well enough to hold my daily lunch, u-lock, glasses and a few other things.
It also has straps to turn it into a fanny-pack style bag.
bicycles.stackexchange.com/users/50/Jack M.
I actually just found this great bag that got high reviews.
Avenir Bigmouth Velcro Seat Bag .
It easily attaches/detaches.
According to one reviewer, it takes less than 15 secs.
This is large enough for my stuff and a few essential tools that were recommended here in this forum.
bicycles.stackexchange.com/users/105/milesmeow
You might want to look into pannier bags (if you have a rack on the back of your bike).
There are all sorts of sizes, styles, and types of bags, including those large enough for groceries.
I got a pair from Walmart (I can't remember which brand) for about $25 that I use to commute to and from work.
They're big enough to carry all of my work clothes, lunch, and water plus some extra.
They're also water-resistant, which is nice when I get caught in a sudden rainstorm (happens a lot in Colorado).
I always have a spare ziploc bag to toss my phone into, just in case.
My bags hook on to the edge of the rack and have an elastic strap that runs around the bottom of the bag and the "leg" of the rack.
I have a set of bungie cords that strap everything down tight
so they don't bounce loose.
It takes me about 30 seconds to take the bags off, and mine came with shoulder straps, which makes them easy to carry into the office.
Performance Bicycle has a nice selection of pannier-style bags .
bicycles.stackexchange.com/users/28/Jared Harley
I find that handlebar bags are the ultimate in convenience.
From your description, a medium-sized handlebar bag might do the trick if you can mount one.
I love my Ortlieb Ultimate handlebar bag , but it's not a cheap item; here are some others .
FOr maximum convenience, look for something that clips onto and off of the bike quickly.
Most bike shops carry handlebar bags.
bicycles.stackexchange.com/users/8/Goodbye Stack Exchange
I'm looking for one that attaches to your bike and easily detaches so you can take it with you.  I usually ride with a small Timbuktu messenger style bag (really small, it's about the size of a large book) and I carry my wallet, phone, keys and stuff in it.  Is there something for bikes so that I can easily attach and detach from the bike?
What is a good small bicycle bag to store your gear and/or personal things?
bicycles.stackexchange.com
<accessories><cargo><luggage>
bicycles.stackexchange.com/questions/205
bicycles.stackexchange.com/users/105/milesmeow
Saddle bags, seat bags, pannier bags and handlebar bags are all recommended for carrying gear when cycling. The range of bicycle bags includes different types, such as saddle bags, pannier bags and handlebar bags. The Timbuk2 Handlebar Bag and the Avenir Bigmouth Velcro Seat Bag are among the recommended products.
The range of bicycle bags includes different types, such as saddle bags, pannier bags and handlebar bags. The Timbuk2 Handlebar Bag and the Avenir Bigmouth Velcro Seat Bag are among the recommended products.