Nanophotonic structures are a technological platform, which enhance processes by confining light to a small physical space. The spatial configuration of the light is known as a mode. In this thesis nanophotonic structures were used to enhance nonlinear optical processes, support optomechanics and multimode optomechanics, and finally to interface with colour centres. In nonlinear optics experiments, we used Gallium Phosphide (GaP) microdisks to generate second and third harmonic light. The small mode volume and low loss of these structures allow the buildup of a very large optical power within the microdisk, as well as a high degree of spatial overlap between the fundamental and second harmonic modes. This allowed efficient second harmonic generation rates for GaP structures. The optomechanics experiments included in this thesis were performed in diamond microdisks, which represented the first optomechanics experiments in diamond. In later work, we modified our fabrication process such that a laser could be placed at particular wavelengths relative to the mode without thermal effects shifting the resonance. This, combined with optimization that reduced the optical loss rate, allowed for operation in the so called ``sideband resolved regime''. This enabled the demonstration of coherent scattering between photons and phonons in these devices. Expanding on this work, we used a second optical mode in these diamond devices to demonstrate multimode optomechanical effects. This author assisted in demonstrations of wavelength conversion, and the first demonstration of optomechanical multimode amplification in the optical regime. We then went on to realize novel interference effects between widely separated wavelengths of light, which were used to build an optical switch. This same multimode device was used to demonstrate optomechanical pulse storage where the storage time and the phase of the stored pulse were enhanced and controlled using the auxiliary mode. Finally, this thesis details our work towards the manipulation of quantum emitters embedded in these diamond devices through coupling to mechanical vibrations in the device. This platform has the potential to realize a compact system, where qubits could be directly controlled by telecom wavelength light.