It's a good question. And I'm happy with my answer.
It's funny this is brought up, mostly because I thought about a tattoo no less than half an hour ago. Fact is, I'd never get one. Not that I have ANY problem with those who do, I just have no desire to do so. At all.
However, if I HAD to get a tattoo for just a week (or permanently), I'd get a very small replica of Captain America's shield, similar to the one in the included image. Captain America has always been a long-standing hero of mine and the shield symbolizes so much more (both personally and pop culturally) than so many options I could choose to slap on my body.
As to where I'd place it...that's a good question. I'd probably want to go with one of my pectoral muscles. That way, I wouldn't see it all the time or all day, but when I did see it, I'd stop and think about why it's there.
In my head and my heart (and the heads and hearts of many others), Captain America's shield is a sign of strength, courage, bravery, pride, and a true adoration for this country and what those before us have have done to fight for our freedom.
It's powerful. And all of that symbolism can be encapsulated in one small, striped shield with a star in the middle.
That's what I'd go with.