COVID-19 affected child/adolescent activities (e.g., extra-curricular, screen time), along with physical health (PH) and mental health (MH); however, less is known about the relationship between changes in activities and PH and MH in the United States and how these relationships vary by race/ethnicity. To address this gap, data were used from a national survey (Socio-Economic Impacts of COVID-19 Survey) administered May–June 2021 (n = 853). Multinomial logistic regression explored changes in outdoor, school, extracurricular, friend, and screen time activities with changes in PH and MH; interactions explored moderation by race/ethnicity. Results showed increases in outdoor (RRR 2.36, p = 0.003), school (RRR 3.07, p < 0.001), and extracurricular activities (RRR 3.05, p < 0.001), which were associated with increases in chances of better PH. Better MH was more likely for children/adolescents where friend activities (RRR 3.34, p < 0.001) and extracurriculars (RRR 4.48, p < 0.001) increased. Except for extracurriculars, heterogeneous relationships were observed (e.g., increases and decreases in activities were simultaneously related to better and worse health). The relationship between outdoor activities and screen time with health were moderated by race/ethnicity. Findings support facilitating outdoor, school, extracurricular, and friend activities, which were positively related to health. Given heterogeneity and variation by race/ethnicity, more research is needed to understand the complex relationship between activities and health during COVID-19.