Thousands of visitors have flocked to Hong Kong’s East Dam to view its rare volcanic rock columns at the start of the five-day Labour Day “golden week” holiday, prompting a lawmaker to suggest charging an entrance fee to maintain the Unesco global geopark.

Drawn by the city’s natural scenery and well-established hiking trails heavily promoted on mainland Chinese social media, many tourists are increasingly trading traditional shopping sprees for ecotourism at East Dam of the High Island...