This research demonstrates enhanced thermal performance in a natural convection-operated thermal system from a fundamental perspective. A simplified but classical configuration of differential heating is studied by employing common base fluids such as air, water, kerosene, and ethylene glycol—all of which are integral to practical thermal devices. Through numerical simulations, this study explores the potential of augmenting the cooling surface area using wavy surfaces or undulations. However, a threshold for waviness-induced performance enhancement exists. Therefore, this study assesses the influence of wavy-walled cooling on thermal performance using different common fluids across varying Rayleigh numbers. The results underscore the efficacy of surface waviness or undulation in boosting heat transfer while maintaining constant working fluid volume and heat load. Optimal improvement occurs with appropriately designed wavy surfaces of limited waviness. The use of a wavy cooling surface can potentially enhance heat transfer by up to 19%; thus, offering a significant improvement in a natural convection scenario. These findings hold significant practical implications for optimizing and designing thermal systems across diverse applications.