BuddyPress.org

@themeanwhile – you can enable or disable the various components so if you want Activities only, enable that and disable other components. After you activate BuddyPress, go to the new BuddyPress menu item on the left column of dashboard, and go to

a) General Settings – these are your choices
Base profile group name:
Full Name field name:
Disable BuddyPress to WordPress profile syncing?: Yes No
Hide admin bar for logged out users?: Yes No
Disable avatar uploads? (Gravatars will still work): Yes No
Disable user account deletion?: Yes No
Disable global forum directory?: Yes No
Disable activity stream commenting on blog and forum posts?: Yes No
Default User Avatar – For users without a custom avatar of their own, you can either display a generic logo or a generated one based on their email address

b) Component Setup
Activity Streams – Allow users to post activity updates and track all activity across the entire site.
Enabled Disabled
Blog Tracking – Tracks blogs, blog posts and blogs comments for a user across a WPMU installation.
Enabled Disabled
bbPress Forums – Activates bbPress forum support within BuddyPress groups or any other custom component.
Enabled Disabled
Friends – Allows the creation of friend connections between users.
Enabled Disabled
Groups – Let users create, join and participate in groups.
Enabled Disabled
Private Messaging – Let users send private messages to one another. Site admins can also send site-wide notices.
Enabled Disabled
Extended Profiles – Activates customizable profiles and avatars for site users.
Enabled Disabled

I would suggest you set up a test install so you can check the features for yourself.

“Of course, activity filters, profile page, friends page, settings page, etc, would also need to be migrated to display in the backend.”
Why? If you want to make the BP Activity section private, you can add conditional tags for is_user_logged_in …