Who to invite: Generally, the bridesmaids and close female friends of the bride. If including junior bridesmaids, the activities should be age appropriate. Do not invite anyone to the bachelorette party that is not invited to the wedding.
Where to go: Consult the bride on what she wants. Do not force activities on her that she will be uncomfortable with or embarrassed by (of course, what those things are vary by bride!). The cliché of a crazy bachelorette party cruising in a limo from one cheesy strip club to another need not influence your choice in activities. The bachelorette party is a last nod to single life, and for some that is best celebrated with a quiet meal and good friends.
Details to consider: If alcohol will be involved, pay for a taxi or car service. Allow everyone to enjoy themselves without having to worry about getting home safely. Also, the bachelorette party is often scheduled fairly close to the wedding because that is when everyone will be available. With such a busy schedule ahead of all those involved in the wedding (and all the pictures that will be taken along the way), it is prudent to exercise as much moderation as possible.