Holwerd itself is tiny — you can walk from one end to the other in a few minutes. No local transport is needed within the village.
A bicycle is the best way to explore the surrounding Frisian landscape. The flat terrain, quiet dyke roads, and long views over the farmland are made for cycling. Bikes can be hired in Dokkum or Leeuwarden if you haven't brought your own.
For the wider area — other Wadden coast villages, nature reserves, or inland Friesland — you will need a car or be reliant on infrequent rural buses. The landscape rewards slow exploration by bike far more than driving.