I use small gear backpacks for day hiking or sightseeing a lot and find them useful - they are a great balance between carrying essentials and being comfortable (they aren't hot or heavy). They can just about hold water (look and see if there is a pouch and exit hole for a water bladder tube if you want super convenient/not bulky water), some power bars, a camera, and an outer thin rain shell and hat and gloves (or a down jacket that compresses a ton).
In your first pic the full side-zip bag is good if there are a lot of internal pockets with small things in them and you want to be able to unzip and pick out what you want (kind of like a hanging travel dop kit). The bag with the 'lid' is great if you are mainly pulling out jackets and want the bag to be able to expand or contract depending on the contents (and serve as a compression sac as well for fluffy/down stuff for example)
You can buy a separate cheap plastic coated rain cover that is elasticized and will fit over the top/outer face of any of these bags. In general I've found that either built-in or separate bag rain covers are good when it's dripping a bit or there is a brief shower, but they don't protect long from downpours. The water will still slide down your back and eventually soak through the padding against your back given a long enough time. So I'd still have a dry bag inside for just your camera, or down, or anything that really can't get wet.
Oh, and if there is a lot of other stuff that needs to go in the bag on a particular day hike, and it seems too small, there is usually someone with a bigger bag in the party that can share the load if need be...