Leftover from last time: create a file with 10 megabytes of random data using dd. (Hint: use the bs option and count=1)
Then, download http://lunkwill.org/random-bytes, which is 10MB of random data I created using dd. I've hidden a plain text message inside that's at least 20 characters long. Good luck finding it with just a text editor. Use the 'strings' command to find it for you. (You may also want to use wget to download the file instead of your normal web browser.)
Now use od |head to print out the data in the file in a more readable format. od -x is the most common way to use it, since it prints out the values (except for the address in the first column) in hex instead of octal. Also, od -c will show you the printable characters printably.
Tricky part: use od, grep and wc to count the number of FF (bytes with decimal value 255) bytes in the file.
Hint: use --format to print out single bytes in hex, --width to print out only a byte at a time, and check out the -v and -A options.
Today I used od -x with a serial remote control receiver, so that I could see what bytes were output when I pressed various buttons. I used minicom to set /dev/ttyUSB0 to 1200 baud, 8N1, then did control-a then q to exit without resetting the port settings. Then od -x /dev/ttyUSB0 showed me what it heard from the port (at least, after it had heard a few lines worth of data).