Holwerd's obvious day trip is Ameland — that is, after all, why most people are here. But the surrounding Frisian countryside offers several other options within easy reach.
Ameland is the main reason to be in Holwerd. The island has wide beaches, a nature reserve, historic captains' houses in Hollum, and a much slower pace of life. A day trip is very doable; staying overnight gives you the island without the day-trip crowds.
Leeuwarden, the Frisian capital, is about 45 minutes south by bus or car. It has a strong identity — Frisian language on every sign, the Fries Museum, a handsome historic centre, and a growing food and culture scene. Worth a half-day at least.
Dokkum is the closest town, about 15 minutes by car. One of the best-preserved small towns in Friesland, with an intact ring canal, a windmill, and a quiet centre that hasn't been homogenised. Easy to combine with a morning visit to Holwerd.
Lauwersoog, about 25 minutes east along the coast, is the other major Wadden ferry port — this one for Schiermonnikoog. The area around the harbour is a seal-watching hotspot; organised boat trips run from the port.