I would second people's recommendation that one of the best things you can do is gather up all the things you what to carry and bring it to a store with a good selection of daypacks. Find a pack that the gear fits perfectly into (not to big, not to small) and feels comfortable to the person who will be carrying it.
That said, to give you a starting point, if you wife is only carrying things she needs, 20L should be a good size. If you are carrying "just" camera gear and she is carrying everything else, 30L might be a better size. Any pack that would be large enough for an overnight (unless you are committed to going ultralight) would be overkill for your dayhikes. FInd something that is right for the immediate problem and revisit if/when you decide to give backpacking a try.
I would note that The Murmur and the Jam are fairly minimalist. Some people love this sort of design. When backpacking this would be my preference, but for day hikes I typically like a few additional features... the Talon is more my speed.
I have written up some others thinks to consider on my recommended daypacks