I like the idea of a backpack and often use them for days out. Like grandtante, I wouldn't dream of putting my purse in it, though.
And do check through your handbag/backpack and reduce the amount of unnecessary stuff you're carrying. Presumably the aim of the exercise is to relieve your back, so reducing the overall weight is obviously the first step.
Check for nice, wide, padded straps and preferably also that the side which is against your back is reinforced or padded. Remember not to pack anything with sharp edges or an awkward shape so that it digs into you.
Ideally, it should also stand up when you put it on the floor, and not collapse into an unruly shape, but those versions are way more expensive.
Many come with a built-in waterproof cover.
Check the zips and buckles before buying and don't be seduced by lots of outer pockets. They're much more use to you on the inside.