Street culture in Hong Kong manifests through busking, community gatherings, street art and festive parades, transforming limited pedestrian environments into vital spaces for encounter and expression. However, recent regulatory closures have intensified tension between creativity and control, and there is limited understanding of the interrelation between spatial policies and spontaneous practices at the micro-scale. This research explores how spontaneous street culture and informal urban practices promote vitality and social resilience in Hong Kong's public space through qualitative methods including spatial observation, mapping, and interviews across three site typologies: central districts (Mong Kok, Tsim Sha Tsui, Central), Fanling New Town, and Wong Chuk Hang Emerging Art District. Findings reveal distinct behavioural patterns shaped by specific spatial logics. Central districts are characterised by a competitive politics of rotation, where mobile performers strive for exposure in narrow spaces during evening peaks; Fanling New Town uses spatial redundancy in open plazas to support inclusive community gatherings among marginalised groups; and Wong Chuk Hang encourages experimental, non-immediate artistic appropriation through graffiti and traces. Synthesising these observations, the study identifies five recurring typologies of informal cultural practices: expressive performances, interactive artistic interventions, collective community gatherings, spontaneous play, and artistic occupation of industrial space. These typologies collectively illustrate how informal activities adapt to distinct urban conditions. The findings demonstrate that informal practices are adaptive strategies that reconstruct spatial boundaries and reintroduce collective action into urban life, contributing to the broader discourse on urban informality, liveability and cultural sustainability in high-density cities.