For example in my job (quite dull) I automated some data collection, stored the data in a database, created a web app to present data from the database, ran it all on a server etc. None of it was anything my boss would have given me as an explicit task, but it was useful stuff that I could make a case for. It was also stuff I hadn't done before and was interested in learning how to do.
Granted, the weight of crappy project management decisions eventually became too much for me to put up with, but for a while I quite liked going in to work and setting up my mini-projects.