Perhaps a better question is "What can't you customize?". Here is a general list of things an admin can do:
- Re-name tabs (Example: Changing 'Accounts' to 'Companies')
- Re-name field labels (Example: Changing a field name from "Type" to "Relationship")
- Change the columns that are displayed in the list view (Tip: Only show the data you care to see at a quick glance)
- Change the layout of a record (Tip: Only show the most important fields up top, take advantage of the real estate on the screen, no pun intended!)
- Change the color scheme (perhaps to match your organization's color palette)
- Workflows. Although an advanced feature, workflows customized to how your business flows really help automate your day. Which leads to increased efficiency. (Example of a workflow: Get email reminders when leases are going to expire for your tenants, when a Listing is created, create a checklist of tasks that need to be accomplished by a certain person or team).