It is possible to take the commuter train to Södertälje, and then a few steps to the steam ship Eidern that goes to Birka once a day in the summer months. 350kr return for adults, 160 for under 18yrs.

Birka itself is mostly a green grassy field, an archeological site. The buildings, forts, and ships are reconstructions of Viking artefacts. 

If the weather is bad, you will not enjoy the site. If it is a glorious summer day and you bring a picknick and enjoy the boat ride you will have a nice visit.