Few studies have investigated how older people’s daily activities and characteristics of the surrounding urban environment affect their emotions and daily satisfaction. This study fills the research gap by using a paper-based survey and a smartphone ecological momentary assessment (EMA) application (with GPS tracking and pop-up surveys about activities) to study older people’s well-being in Hong Kong. Three hundred and two older adults completed the paper survey and forty-two of them further completed a six- to seven-day EMA survey. Based on a total of 1,530 activity episodes, generalized estimating equation models were developed to examine the relationship between older people’s socioeconomic background, activity participation, urban environment characteristics, and their feelings of happiness and calmness. Results show that older people’s daily life satisfaction is affected by the emotions they experienced throughout the day. In particular, a higher level of happiness was associated with socializing, leisure, and recreation activities taking place outside of the immediate neighborhood, and calmness was best achieved during passive relaxation and resting activities within the residential neighborhood. Furthermore, characteristics of the environment at activity locations are statistically significant in explaining momentary well-being. The findings support the planning for age-friendly cities and communities through encouraging older people to engage in activities outside their residential neighborhoods and designing activity locations with more leisure facilities, lower road density, and higher exposure to water bodies.