Another one of those razor's edges, Rob. In gov, you're right, it's easy to know what to do. A good rule is that if you or the organization gets something of any value, be willing to pay for it. This helps to keep vendors in business and avoids hidden costs down the line.
Big conflict of interest is easy to avoid. If I need to go to a vendor site, surely my org can pony up for a plane ticket if it's that important. If a vendor is sending me for "free", you can bet that my org will be paying for it eventually, most likely with a markup. When I was on the vendor side, I'd frequently hear cackles of "we'll get that money back 10x over." So, ethical considerations aside, why not just pay for it in the first place?
The smaller conflict of interest stuff? A lot harder, but just as important, because it leads to the bigger stuff. Dishonesty flourishes because we enable it.
Pay for your own darn lunch. Go to lunch because you want to, not because there's a freebie attached. Free isn't really free, there will be hidden costs. TANSTAAFL: There ain't no such thing as a free lunch. Make sure you know what the value exchange is.