I think this depends on you going into a bit more detail.
Over the years, I've taught myself to pack pretty light for day excursions from May-Sept. This (for me) means choosing clothing that'll leave me a bit cold in the morning to be comfortable by midday (no layers/jackets/etc), that'll also dry quick (which eliminates rain gear).
I'll eat a decent breakfast before I get started for the day, and I'm generally good on food then until I get back that evening, skipping lunch, so at most I'll take along a dry granola bar.
Unless I'm kayaking, I only carry one rod, which eliminates the need for a tube for me.
Water is really the only concern...and even then, I think there's something mildly wrong with me, because I'll start getting sick from dehydration without realizing how thirsty I am, so I'll generally go 6-10 hours with either a single 20oz bottle, or nothing at all. That has gotten me into trouble before though, so I try to keep myself better hydrated, and I'm on the casual lookout for a good filter bottle that'll let me fill it straight from the stream.
This all in mind, my 6-10 hour pack isn't much more than my 2-3 hour pack: my sling with my fishing gear, with a water bottle and a granola bar thrown in.
This year, I'm going to try and integrate a small hydration pack (a Hydrapak Laguna worn under my waders, so as not to interfere with my sling) to keep myself better hydrated. If the whole getup is too cumbersome, I may ditch the sling for long trips, and just throw a few fly boxes in the hydration pack with a tippet spool and nippers clipped to a shoulder strap. In addition to the bladder sleeve, it's got several compartments and pockets for storing small items and should be perfect for day trips.
