A huge expanse of fascinating lake lies just to the west of Stockholm. Many people get just a taste of it rushing between Södertälje and Stockholm, but we spent some more time exploring it. Despite the weather we had, its a great area. There are many guest harbours, fewer, but still interesting nature harbours, and a few large towns. The character is different from the archipelago, with many islands with the trees coming right down to the water and less boat traffic. We visited early, where everything was open, but often we were the only visitor.
Mälaren is criss crossed by a number of bridges. Pretty well all the fixed bridges are at 25m height, except for an 18m one at Staket, on the way to Sigtuna. The rest lift, usually once an hour, from about 0700 to 2100 or more. The opening times are specific for each bridge. You need the Upplev Mälaren guide to tell you when they open. You can however talk to the bridge operators on VHF.
You can access Mälaren either via the Södertälje canal (one lifting bridge, one lock), or via the Hammarby waterway from Stockholm Harbour (two lifting bridges, also one lock). Cost is currently 140 SEK, paid on exit.