If you can transmit light through a sample curvette to the spectrometer, you could use it to measure optical density (i.e. the amount of stuff in the way that b

If you can transmit light through a sample curvette to the spectrometer, you could use it to measure optical density (i.e. the amount of stuff in the way that blocks or scatters light). You could use that to measure the concentration of particulate in engine oil, or the strength of coffee. If you can detect at 405nm, you could measure blood coagulation time/extent (separate blood cells from plasma, measure attenuation at 405nm though plasma). You could cludge together a pulse oximeter if you can detect at 660nm and NIR (905, 910, 940nm) and have light sources to emit at these wavelengths. Absorbtion at these wavelengths changes as hemoglobin picks up and loses oxygen. The ratio at 660nm and one of the other wavelengths could be used to show blood oxygenation. If you were into photography or cinema, you could measure the emission spectrum of a light source and fit a blackbody curve to it to find its color temperature--useful for white balancing. If you wanted to match paint colors, you could use a spectrometer to do (along with some additional calculation and knowledge about your pigments). If you perform a flame test, you could use a spectrometer to learn about elemental composition.