Amateur Radio is a hobby that has a lot of depth to it. There are so many facets that an operator/enthusist could explore. Here is a list:
HF: Using shortwave radio bands to communicate from 160-10 meters to talk regionally or long distance.
DX: Talking to radio operators who live in distance locations and countries.
Beacon: continuous transmitting station on a specific frequency for use in determining band openings and radio propagation quality.
VHF/UHF: Using higher frequencies like 2-meters or 440 MHz to talk to other radio operators locally.
Repeaters: setting up repeater radio with an antenna on a tower or mountain that facilitates communications throughout a local area. Repeater linking lets operators talk with operators in a neighboring location.
VOIP: Repeaters can be connected by the Internet allowing operators to connect to other EchoLink repeaters all over the world.
Satellite: using VHF/UHF to contact ham radio satellites and connect to operators in distant lands. Talk to astronauts on ISS.
Moonbounce(EME): using high power VHF/UHF to bounce radio off the moon and make distant contacts using line-of-sight, local frequencies.
Homebrew: building your own equipment.
Antennas: designing, constructing, and using your own antennas: wire, beam, vertical, phase-array.
Radios: building your own radio receivers or transcievers from scratch or kit.
Powersupply: building your own power supplies.
Antenna Tuners: building your own antenna tuner/match
Amplifiers: building your own amps.
Event Stations: set up a radio station during an event and publicize the event to other radio operators.
Contesting: participating in contests to see who can make the most contacts.
Paper Chasing: getting ARRLs awards for "worked all states", "worked all continents" etc.
QSL Cards: Sending and receiving colorful postcards that confirm radio contact with operators all over the world.
Digital Modes: Communicate via CW key or computer. Morse Code, Packet, Fast and Slow Scan TV, Teletype, PSK, Spread-spectrum, etc. Receive E-mail over the radio using an Internet-linked Gateway and TNC.
Software: design software to interface with and control your radio or to send digital modes over the radio.
APRS: automatic packet reporting system. Send and relay your GPS position or set up a weather station that sends telemetry data.
Community Service: provide volunteer radio communications for large events like marathons, triathlons, bike races, and parades.
Disaster Response: participate in disaster drills and provide communications in times of emergency.
Mobile Radio: use HF/VHF/UHF radio from your car, plane, boat, or hot-air balloon.
Emergency: use battery, solar or other alternative power in preparation for emergencies.
Weather: Skywarn nets pass information on storm conditions directly to National Weather Service.
QRP: low power radio communications < 5 Watts.
Direction Finding: Activity were operators race to locate a hidden source of radio transmission using a directional antenna.
Nets: Organize and hold weakly or daily nets on HF or VHF frequencies bring together radio operators with similar interests.
Trade: Attending HamFests, swap meets or traders nets and buying, selling, or trading radio gear and related electronic equipment.
Vintage Radio: restoring, collecting and operating antique radios.
Education: teaching ham radio classes and/or being a VEC or volunteer examiner. Developing learning materials, courses or teaching devices.