Old Postal Route of the Swedish kingdom.
Nice enough small B-roads on the islands with some very beautiful spots, on Aland one of the main A-roads with several interesting sights (Bomarsund fortress ruin, Kastelholm castle, Uffe pa Berget hill). Rather expensive route, as Alandstrafiken had a surcharge (a stiff 100EUR in 2012) to discourage transit traffic, and ferries quite busy on summer weekends. More suitable as a mini holiday for at least two if not three-four days, to savor the scenery, live a bit of the island life, and visit places (say, the Maritime Museum in Mariehamn with it's Flying P Line windjammer Pommern), as you'd need to hurry to make the ferry connections in a single (full) day. Longer connection from Torsholma (Brando) to Hummelvik (Aland) makes stopovers at islands, mid-size ferry with a cafeteria, quite nice. Could be an alternate route between Sweden-Finland if you wish to choose the slow boat and can take the cost, and take the fairly nice but a bit tedious rides btw. Stockholm-Grisslehamn and Osnäs/Gustavs-Turku, for the whole Postal Route.
No wild camping on the islands unless the landowner permits, no camp fires (high risk of forest fires on the bone dry islands). Sensibly low speed limits, considering the bursts of traffic from ferries, a lot of cyclists, some pedestrians. Asphalt has a peculiar reddish tint, mostly good save for some rough spots. Very scenic stretch when passing from Ava to Brando on an embankment over small islands, but otherwise mostly inlands thru forests and the odd pasture land. Weather often fair out in the archipelago even if there are rain showers on the mainland, afternoon sea breeze, hot (>25 C) calm days rare, sea water temperature refreshingly in the teens/high teens later in summer. Expect mosquitos on calm evenings and nights, "white nights" in the weeks around the summer solstice.
Ferry timetables: alandstrafiken.ax/en (Northern Route), eckerolinjen.se (Sweden-Aland)
Take a trip trough the Aaland islands norhtern archipelago, whith more than 1000 islands, very nice nature and some really nice roads!