This study investigates natural convection in shallow rectangular cavities filled with non-Newtonian Al2O3 nanofluids based on water–ethylene glycol (W/EG) mixtures. Analytical and numerical approaches are developed to study the impact of nanoparticle volume fraction (0 ≤ φ ≤ 0.05), power-law index (0.6 ≤ n ≤ 1), and base fluid mixture on heat transfer performance. The governing equations are solved numerically using the finite difference method, and analytically using a parallel flow approximation. Both approaches demonstrate excellent agreement of results, which validates the analytical model for high-aspect-ratio cavities. It is found that the increase in φ or ethylene glycol concentration results in significant viscosity improvement that deteriorates both the flow and heat transfer rate despite moderate improvements in thermal conductivity. The Nusselt number decreases drastically with φ, especially for EG-rich mixtures. Convection is also reduced by increasing n, but to a lesser extent at already high viscosity.