You may want to add which country laws you're trying to adhere to. From my rather limited experience even I know that the laws for radio transmissions are different in different countries of the world.
FRS - Family Radio Service No license is required but legal operation requires the use of unmodified type-accepted transceivers.
ISM band 902 - 928 MHz
ISM band 2.4 - 2.5 GHz
ISM band 5.725 - 5.875 GHz
ISM band 24 - 24.25 GHz
Frequencies greater than 275 GHz
Keep in mind that unlicensed does not mean unregulated. Most of these bands have power level restrictions.
Also note that if you are interested personally in data transmission, then there are a lot of bands that having an amateur radio license opens up access to.
Infrared, visible, and ultra-violet light will be the major players. When I was in high school (Lord, back in '88), I made a pulse frequency modulation laser-diode comm set that would transmit a couple kilometers. You also have the option of ultrasonics, along the line of a Gertrude, where you can heterodyne an audio signal to upshift it in frequency. You can get surprising range from this.