OK, so you'll probably have to pass through here if you come to Åland, but move on quickly. The centre isn't quaint, it's commercialized. Boring tourists doing boring touristy things. To truly appreciate a visit to Åland, head out into the countryside. Small villages and middle of nowhere spots are the real Åland.
Unique Suggestions: At least try some local food and pick up some Åland stamps. And by local food I don't mean Hesburger. They have local cheese, local pancakes, and local beer (Stallhagen Pale Lager, not Åland Öl, which is made on the mainland). If you must spend time in Mariehamn, eat something local.
Fun Alternatives: Alternative - get into the countryside. It's the only way to see Åland.
Getting to the Aland islands (Finland) from Grisslehamn (north of Stockholm, Sweden) is really easy and cheap, BUT, be careful if, as I had done, you plan to go just for one day, especially on saturday when you imagine being somewhere during the touristic season on a saturday will offer you plenty of things to do... The Eckero-Lijnen have 5 go-and-back everyday. The first is scheduled at 10:00am. You arrive 2 hours later (Time changes between Sweden and Finland). Drive quickly (but securely !!!) to Mariehamn that is about 33km far from where the boat arrives: because shops close at 2:00pm !!! If you start your one-day-journey visiting the Postal museum in Eckero, you will, afterwards, arrive too late to have a lot of time to enjoy the city of Mariehamn. Hopefuly, the Pommern (a great ship) should still close later than 2:00pm and rambling in the streets and in the harbours is always fine abroad !
In the Aland islands, you're in Finnish territory, but everyone speaks Swedish (and english, don't worry !)