The National Archaeological Museum Aruba (NAMA) is the most comprehensive place on the island to understand the pre-Columbian past. The collection covers the Arawak people who inhabited Aruba from around 1000 BCE, with displays of ceramics, shell tools, stone implements, and skeletal remains found at excavation sites across the island. Explanatory panels in English, Dutch, Spanish, and Papiamento make it accessible to most visitors.
The museum is housed in a handsomely restored colonial building dating from 1929, originally used as government accommodation and later as a court. It sits in a quiet part of town away from the cruise-ship shopping strip, and admission is free. Allow an hour to do it justice. If you're going to visit only one museum in Oranjestad, this is the one to choose.