Hi — Here's my 2 cents: (based from what I learned from my current game making)
Idea: Any. Don't sweat searching, ideas will come to you. (take some time)
To be simple: Try the game idea you think it's a bit too simple for you. Try it. Often 'simple' games can take more effort than one can imagine. After the basic structure is done, built around that "simple" game, don't sway too far or aim too high, keep only a few game features, & keep it simple!
To be enjoyable: I don't think there's an easy way for this. You have to find your way how to excite & engage the players, or how you present your game. Every single thing in the game can have an effect: making this background more beautiful, adding this code so players won't be confused, adding this punchline because it's FUNNY, etc. There will be hardwork & dedication.
Also, lots of feedbacks helps polishing the game to make it even better.
Hope this helps.