Good Contests and Activities to do in High School:

I frequently meet the college students who missed out on an incredible activity in high school simply because they didn't know about it. Let's remedy this. Also, check out WhyIsItAwesome, a wiki that explains how to get into things people are into.

Awesome Activities:
Contest | Prestige | Description

Academics:
Quizbowl | Medium to High
Forensics | Medium to High

Business:
DECA | Medium
FBLA | Medium

Writing/Literature:
Live Poets Society of New Jersey National High School Poetry Competition | Medium | Submit a poem, 20 lines or less
National Council of Teachers of English | High
National YoungArts Foundation | High | National writing (or dance, art, cinematography, etc.) competition
Scholastic Art and Writing Awards | Low to High (depending on placement) | Writing competition that ranges from regional to national level
Publication in a literary magazine | Medium to High (depending on magazine) | Winter Tangerine Review, The Adroit Journal, PANK, DIAGRAM, Word Riot, Vademecum Magazine, The Postscript Journal, Killing the Angel, etc.
Foyle Young Poets of the Year Award | Medium to High | International poetry competition sponsored by the U.K. National Poetry Foundation

Science:
Intel Science Talent Search | Very High |
Siemens Competition | Very High |
USNCO (US National Chemistry Olympiad)/IChO | Medium to Very High
U.S. Physics Team/IPhO | Medium to Very High
USABO (USA Biology Olympiad)/IBO | Medium to Very High
RSI | Very High | Summer research program
Simons/Clark/HSHSP/Garcia | High | Summer research program
SSP (Summer Science Program) | High | Summer program involving astronomy, physics, programming, etc.
Science Olympiad | Medium to High

Math:
AMC (American Mathematics Competition)/AIME (American Invitational Mathematics Competition)/USAMO (USA Mathematics Olympiad) | Medium to Very High |
ARML | High |
HMMT (Harvard-MIT Mathematics Tournament) | High |
UCSD Mathematics Competition | Medium |
PUMaC (Princeton University Math Competition) | High |
Math Prize for Girls | Medium to High | Requires AMC cutoff score
USAMTS (USA Mathematical Talent Search) | Medium to High | Participants are allowed several weeks at a time to work on 5/6 proofs.

Computers:
ACSL (American Computer Science League) | High | Relatively easy but major, nationwide computer science contest. With a bit of study, it's not overly difficult for your school team of three or five to make nationals. 4 yearly tests with written and programming portion.
USACO (USA Computing Olympiad) | Very High | Rigorous and challenging, but very prestigious algorithms-based computer science contest, leads to the IOI (International Olympiad in Informatics, Analogous to the International Mathematics Olympiad Computer Science). Contests are taken individually at home. Regardless of whether you're interested in competing, online algorithms training program is incredible for general knowledge/edification/college.
Botball | High | Fully autonomous robotics contest with fixed pieces, Legos and other
FIRST | Very High |