Speaking of 5E monsters, anyone who plays that edition of the game and enjoys monster creation should consider entering the Monarch of Monsters contest.
Anyway, the spreadsheet! Let's have a look.
Stat blocks in D&D 5E are quite concise, which makes it easy to place most controls on one page. On the right, you can see the interface for entering the monster's stats. Green boxes mean "insert a number" and blue boxes mean "insert text" or "check a box". Everything else is generated automatically.
On the left, you can see the stat block being generated in real time. If you check the Deception box, for example, you'll immediately get Deception +3 on the Skills line. Any abilities you haven't added yet won't be shown. Some of the selections are available as drop-down lists, so you easily pick a valid value.
Whenever I'll find the time to develop the spreadsheet further, I'm planning to also add gauges (think guitar tuners) that show how close you are to the target values for the Challenge rating you've picked.