1) Sizes - observe their apparent (angular) size, use their known distance to get their actual size.
2) Masses - observing their gravitational effect on some relatively lighter body, e.g., on a natural satellite, or on a comet or asteroid which passes nearby.
3) Densities - The "density" of an object is its mass divided by its volume.
4) Rotation rates - either by watching some surface feature move as the planet rotates or by measuring the difference between the frequency (Doppler) shifts of radar reflected from the two opposite edges of the planet.
5) Composition - directly, with a spectroscope, or indirectly, from density; for example, average density of Jupiter is 1gm/cm3 , therefore, it could not be mostly iron, since the density of iron is about 8gm/cm3 .
6) Atmosphere - detect by occultation observation; also, spectroscopically.
7) Albedo - fraction of the light incident upon them which they reflect away.