Here is a quick paper (notes) about pen-testing a Windows Active Directory network. While I do know this paper covers only the lowest-hanging fruit, it seems that all too often, these lowest-hanging fruit are the most common fruit found in the wild.
I will add to make sure and grab the cached logins on the workstations attacked as well. Often, systems cache the default last 10 accounts, which almost always includes at least one admin-type account from desktop support or the person who made the image in the first place.
If you crack the local admin password, don’t just use it on other systems, but try to change obvious things in the password. If they’re not the same across the department or even the company, often desktop support has some sort of predictable password scheme based on the computer name or user name or department. Heck, even I had a predictable one back when I did support, but you really had to work to guess it and I left plenty of red herrings laying around (like having the second half of the hash crack into a known word or just lower-case letters to throw off how complex the first half was…)