Dental Floss is the Swiss Army knife of travelers tools.  I tape a few sewing needles to the outside of the dispenser.  I've repaired luggage, shoes, and clothing, made shoelaces, used it for twine, used it as a cheese knife, and even used it to clean my teeth.  I once added added a grab handle (from a sacrificial convention bag) to a Samsill (Microsoft) case while stranded in an airport hotel for a day (boredom is the godmother of invention).  Its still there today (and used frequently for my presentation days and one night trips).  It looks pretty bad but works great, I just wish they made floss in black.   Its not truly a hack but, like duct tape, an indispensible part of making do on the road. 

I've added strips of the loop (soft) side of velcro to the insides of my most used luggage and dots of the hook (hard) side to one side my folders and packing cubes.  I find that by 'fixing' the folders and cubes in place that my clothing is less wrinkled and the bag retains it shape better even after being coaxed into an overhead or under a seat.  Using the soft side in the bag allows you to pack anything in the case without damage that the hook side might cause.

Nothing smaller, lighter, or cheaper as a water bottle than re-using an old disposable one. My favorite is the "overbuilt" Gatorade-style bottles (found on Gatorade but on many other drinks like Vitamin Water) with the ribs on the side to reduce flexing. At the other end of the spectrum, Costco recently made the plastic on their bottled water bottles even thinner and the cap half height, if you are counting grams. :-) But any of these is lighter than a Nalgene bottle.

You can also find "half height" bottles, some with wide mouths, which won't allow you to keep up with a camel across a desert, but will allow you to take a good drink from a water fountain (much more efficient than trying to "gulp" at the stream of water coming out. And of course a shorter bottle will fit under the tap in the washroom sink, for those situations were there is no water fountain.

Till:  I used both.  I used TearMender ( a fabric glue that works quite well) to position the strips where I wanted them and once the glue set, I achored the strip with a stitch through the lining (I flexed the lining away from the outer nylon) upe each long edge and across the ends.  By going through the lining alone there is some give between as the bag moves around. I use small squares of the hook side so I get a modest attachment, not a deathgrip.  Using long strips of the loop side allows me to adjust the location of the cubes anywhere in the bag I have four 20" strips running the length of both sides of my current 22" case from 5.11 Tactical.

I find the velcro works well in the cases where I'm not filling the bag, giving the bag some structure and supporting the clothes.  This was a real problem with my old Eurosak (a bag much like the OP but less well thought out).

As I write this, it becomes obvious that I've spent way too much time thinking about this stuff.