<p>Cardiopulmonary exercise testing (CPET) is a cornerstone of functional assessment in congenital heart disease (CHD), yet there are additional important contributors to exercise limitation that are not directly measured by CPET. This narrative review highlights the complementary utility of spirometry, body composition analysis, and evaluation of handgrip strength in augmenting our understanding of functional capacity in CHD. Spirometry provides insights into restrictive lung disease patterns prevalent in post-surgical CHD populations; body composition analysis, especially lean mass assessment, can reveal sarcopenia not apparent using BMI alone; and handgrip strength serves as a validated surrogate for overall muscular function and frailty. We propose routine integration of all three testing techniques as complements to CPET protocols to enhance clinical assessment, guide rehabilitation, and support prognostication in pediatric CHD patients. This review advocates for broader adoption and standardization of these measures in clinical settings to improve risk stratification and long-term outcomes.</p>