Here are some more things you can do with your RTL-SDR after the first 50:
Meteor weather satellite reception (Russian counterpart of the NOAA satellites, but digital, so higher res and in color)
Digital Radio Mondiale -- digital radio but for shortwave
Analog TV -- if you're in an area that still broadcasts this (unlikely), you can receive a black & white picture and closed captioning. If no OTA broadcasts remain, you can use the analog output of a VCR or DVD player
GPS -- rtlsdr is capable of decoding GPS, Galileo, and BeiDou! (Likely not GLONASS since each satellite uses a separate frequency, spreading the signal beyond the sdr's bandwidth)
Hidden secondary audio broadcasts inside FM radio (like the stereo audio hack, but using higher frequencies in the demodulated stream)
Brazilian outlaws and UHF pirates using open repeaters on US military satellites launched in the 70s
TEMPEST / "Van Eck phreaking" where you can remotely read a nearby screen due to leakage from the monitor or video cabling
Instrument landing system -- if you're near an airport you can tune to a runway's ILS frequency and see the signal change as you move from the left side of the runway to the right
Infrared remotes -- stick an IR photodiode in the antenna port and you can demodulate codes from remote controls
Passive radar -- Tune into a very narrowband signal like a VOR or ATSC pilot signal, set your decimation extremely high (i.e., trading bandwidth for dynamic range) and you can see nearby planes in the area from their doppler-shifted reflections of the main signal