When thinking about a day out shooting, consider putting a schedule together to take into account the direction of the light compared to the subject.
Points to consider, where will any people be in relation to the subject, you do not want them or telegraph poles, wires etc. in a background for example, especially during activity. If you can work with the early morning or evening light when it is softer and avoid the harsh mid-day light unless you are trying to make things difficult for yourself..
The use of fill in flash can be used at any time of day, if shooting early in half light try starting at minus 2 EV on the flash unit, then as the light increases so you should increase the amount of light by 1.3rd to ½ a stop until full light is used.
With TTL metering this is a lot easier than it used to be, remember if you can raise the flash above the lens to avoid those devil eyes (red eye)
How many of us seem to have everything in our bags we have bought and really never use on a day to day basis? Before going out decide what it is you are going to shoot? I.e Birds? large telephoto, so why carry your macro, reflectors, ground pod etc. The inverse being true. The constants being Flash, cleaning cloth, blower, rain cover!! (we are in UK) release and a filter or two. It also stops you worrying about things you have had to leave in you car.
Using a photo vest will certainly assist in carrying the smaller things like spare battery, cards, hip flask!! And god forbid the camera manual!!! A good light rain cover for those English showers, pinch the shower caps from your hotel, you know those plastic throw away ones. They cover the camera brilliantly and you can see through them.