Stage 1 of the Baltic Sea Cycle Route invites you to Ærø by ferry from Sønderborg and further on to Svendborg (or vice versa). But there is actually a fantastic alternative to this ferry route: ÆrøExpressen, which sails between Rudkøbing on Langeland to Marstal on Ærø.
If you take this shortcut, you will see a completely different part of Ærø. On top of that you save a few kilometers on the bike by "skipping" Tåsinge and Siø.
If you end up falling in love with Ærø - which is a very common crush - you can stay on the island another day and go for a bike trip all the way around Ærø. The trip is available in two versions of 60 and 100 kilometers.