We visited BIRKA this last august and we get just a bit disappointed... if you are from south Europe and you are used to see ancient buildings (greek, roman...) you may use your imagination so much in Birka as there is nothing to see, but a wonderful countryside and extraodinary museum and museum-guides.
Just remember everything was made of wood...
Fondest memory: wasp! if your intention is to enjoy a relaxed pic-nic... you will be chased and persued by 1.000.000 wasps and insects!!