I have painfully come to the realization that a bag large enough to hold all of your contents is too large to carry around comfortably. So my #1 advice - get a small bag. An excursion or day bag is useful for carrying just the items you need for the afternoon.
If you need to carry more equipment, you can carry a larger camera bag as well. If I am not using my Mother Ship solution, what I sometimes do is empty out the excursion bag, putting all of the contents into the larger camera bag for transport to the ship. I then pack the excursion bag in my main luggage, with socks or underware in it so that it doesn't take up too much space.
Style: sling/belt pack
Size: 11"W x 4.375”D x 9.5”H
Weight Empty: 1.6 lbs
Weight Full (when I'm done): 8 lbs
Features: chest straps
Street Price: $130
I now use a Think Tank "Change Up" bag for day use. It is normally a sling bag, but it can be used with a pull out waist belt that is permanently attached (but stuffs into the bag when not needed). The belt has loops on it so you can attach pouches if needed. The bag is large enough to hold a DSLR with lens attached, a 11-16mm super-wide, and a flash. There is a front pocket for accessories - spare batteries, and such - and even can carry passports. There is also a protective rain cover inside of it as well.
A short video describing how I use the Think Tank Change-Up
The bag can be used as a belt-pack, shoulder bag, or used with a chest harness.
The Change-Up comes with two straps so that you can use both for a chest strap (with a cross-over at your back), or by using a single strap, it functions as a shoulder bag. However, I am a tall guy and the straps are just a bit too short for me. As well, at 7lbs fully loaded, the bag is just about too heavy for the supplied straps. It's not that the straps aren't heavy enough - they are narrow and without any padding on them, they are uncomfortable.
I normally use the Think-Tank Change-Up as shown here with the Skooba strap. This strap has a suspension which is created with "bungee" cords.
Scooba Design Bungee Shoulder Strap
Converting the Change-Up to belt-pack use is easy. Simply pull the permanently attached belt sides out from the backside of the bag as shown below. The belt sides are thin enough that they do not bulge the bag in any way; a very convenient feature.
After the belt sides have been pulled out, you can attach modular pouches if needed to the front side to carry extra gear. But the caveat here is to not go overboard, or you are right back to lugging around more gear than actually needed.
While they are a bit expensive, Think Tank products are professional grade, and do carry a lifetime warranty. They should provide years of enjoyment.