Click here for an Android Police article that explains how you can still use your Chromebook when it is not connected to the internet.
Click here for a really excellent educational video by a vlogger called Eevnos that introduces you to the capabilities of ChromeOS [with particular regard to running applications normally found on other popular systems] both when on-line or in off-line mode [when internet connectivity is not available] making it almost as versatile as a traditional laptop.
Eevnos covers Native Chrome OS Apps, Android Apps, Linux Apps and SOME Windows Apps - and because there is a lot to cover he kindly provided this index :-
00:00 - Introduction
00:41 - Native Chrome OS Apps * See Applications paragraph below.
02:05 - Android Apps
04:39 - Linux Apps
09:05 - Installing SOME Windows Apps
15:32 - Other options and wrap-up
N.B
You will also find the above index just below the video frame on the Youtube page together with a lot of other useful information that is worth reading, including links to other resources.
Also Eevnos does point out that not all of these features will run on older, lower specification, Chromebooks. However if you are fortunate enough to own a more powerful model then a lot of what is shown will be available to you. Enjoy !
Click here for a canned search for offline enabled Apps currently available in the Chrome Web Store meaning they can be used on any Chromebook when there is no internet connectivity available.
Click here to visit the Google Play Store where you can download Android Apps to your Chromebook - in this case it shows the results of a canned search for Sudoku showing over 250 examples for you to choose from.
Click here for the Platypus site's offline games page article that lists the current selection of games that can be played offline.