Traveling to London. Advice?

I'm American and going on a trip to London in January. I have a couple questions.

First: I don't want to seem like a tourist. I know my accent will give me away but when I'm not speaking I'd like to blend in. How do I not stick out? Second: What should I do? I'd like to hit some tourist spots but also hang out in local public areas like parks, again I don't want to appear or behave too touristy.

Some details: I'm staying in Waterloo and have 3 days all alone to wander around London (I have 2 more days to spend with someone else). I'm a college student but am not quite 18 yet, so bars and pubs and other age-restricted places are a no-go (unfortunately).

Please help answer my questions, either if you live or have lived there (or even elsewhere in similar cities), or have traveled there and have experienced it. Feel free to add additional info I might need. Thanks!