I use my bicycle for getting around town, running errands, e.g., going to the library, grocery shopping, etc. Over the years, I have found that a permanently mounted bag on a bicycle has served me best as a bicycle bag. It is non-removable and stays in sight ll the time while riding the bike. You need a small front rack mounted on the brake bosses of a mountain bike.
This instructable outlines a way to attach the backpack to the bicycle. There can be many variations of the same based on your bike and backpack.
The first bag I used was a discarded school backpack with a Chicago Bulls logo. It was my son's bag when he was in elementary school. Unlike the current times, it was a well made bag and served many years and was the right size for a front bike bag. I had to replace it this year as it was fraying in places and was not water resistant any longer. This was attached to the bike front rack at the bottom and I had tied the back straps around the headset tube and handle bars to stabilize the top.
Step 1: Prepare the Backpack
The current version is a cheap ($5) backpack from a discount store. I had to carefully cut off the straps very close to the seams. I could not tie the straps like the first version as even after tying, the top of the backpack was not staying upright.
To secure the bottom, make a stiffener from a piece of corrugated plastic sign or similar lightweight but stiff material. You may also need a stiffener for the back. Mark the holes depending on the rack and make the holes using a heated nail to keep the synthetic fibers from fraying out.
Make a bracket of some sort to secure the top. In my case it was a piece of metal with holes to attach it to the stem at one end and the backpack at the other.
Step 2: Mount on Bicycle
The bottom is secured with nuts and bolts salvaged from discarded dish antenna. The bottom is stiffened with a piece of corrugated plastic from lawn sign cut to fit the bottom. Also, use large flat washers (fender washers) to cover as much area as possible to avoid stressing and tearing the fabric around the bolt holes. The bolt holes are best made using a heated nail to seal the hole and stop fraying.
Step 3: Load and Ride
I use the main compartment to keep a small pump, a tube, work gloves and some other tools. These stay at the bottom of the backpack in several plastic shopping bags to stop rattling. The space left on top is used for carrying books, groceries, etc. depending on the ride.
The front pocket is used to carry the bike lock key, wallet and phone on the bike. When I have to leave the bike, e.g., going into a store, I just transfer these items from the front pocket to my pants pockets which I like to leave unloaded while riding. When I come home, the wallet and the phone are removed but the key stays on the bike.
So far, no one has touched the bike bag while the bike was locked outside the stores, libraries, etc and the contents (pump, tube, etc) were left alone.