Ærø is roughly 30 km long and 8 km at its widest, gently rolling rather than flat, and laced with quiet country lanes and a handful of signposted cycle routes. Cycling end to end — Søby to Marstal via Ærøskøbing — is a comfortable day's ride with several stops, and the National Cycle Route 8 covers the spine of the island.
You can rent bikes at the ferry terminals in all three towns and at several pensions; e-bikes are widely available and worth the small extra cost on the hillier sections. The countryside is open and unfenced, with stone-walled fields, thatched farmhouses, and frequent views of the sea. Pack a swimming costume — beaches are never more than a few kilometres away.