The former sultan's palace, built by the Dutch in 1937, is now a decent museum chronicling the culture, natural history and industry of Indonesia's oldest kingdom – as evidenced by 5th-century Sanskrit engravings (the originals are in Jakarta). The ornate Yuan and Ming Dynasty ceramics are compelling, while the wedding headwear from around Indonesia is entertaining. The architecturally incongruous building is itself an attraction, with strong parallel lines reminiscent of Frank Lloyd Wright's work.
Don't miss the exquisitely decorative wooden palace out back, which stands empty outside of the Erau Festival, when the current sultan takes up residence.