In this work, we experimentally demonstrate the phenomenon of nonlinearity-activated intermodal tunneling in a periodic elastic metamaterial waveguide with internal resonators and we show how this effect can be exploited to achieve conspicuous energy localization and trapping. The architecture of the waveguide is deliberately designed to promote tunneling from flexurally dominated to axially dominated modes, in order to accentuate the functional complementarity that can be harnessed during tunneling. 3D laser vibrometry at different scales of spatial refinement is employed to capture global and local in-plane features of the wavefield. The measured response naturally yields an experimental reconstruction of the band diagram of the waveguide and reveals unequivocally the spectral signature of the high-frequency modes that are activated by tunneling. Finally, a detailed scan of selected cells highlights a strong and persistent axial activation of the resonators, which displays subwavelength deformation features that are unattainable, for the axial mode, by exciting at the same frequency in a linear regime. This result demonstrates the viability of tuning strategies based on nonlinearity and paves the way for the design of metastructures with enhanced energy-trapping and harvesting capabilities.