Mariehamn is very walkable (and even more bikeable), but you may want to hire a car to explore other parts of the archipelago. The friendly RBS Biluthyrning is at the St1-garage petrol station opposite Mariebad; rates for small cars start at around €82 per day. Book well ahead in summer.
Ro-No Rent hires out bicycles, mopeds and other recreation gear.
For a taxi, call 018-26000. Water taxis take passengers out to Kobba Klintar and other nearby islands.