I'm making a 30th birthday cake for a friend this weekend. I would like to make a 30 out of either royal icing or fondant to use as the cake topper. Any suggestions for the best way to "attach" either to the top of the cake? I've used toothpicks before- but that was for straighter numbers- 1 and 6. TIA!

I've used straws, popsicle sticks, dowels, just about anything depending on the size, height, etc. I've either stuck them in while making them, or attached them to the back of my figure with royal.

I recently used the coffee stir sticks/straws...they are food safe and remarkably sturdy. Plus they are easy to cut

If I only stick the straw in about 1/2" into the number will it be sturdy enough? I can't figure out how to make a 3 or 0 with a thicker "bottom" without it looking funny.

Sure you can stick it just partway up one size of the zero, and the three...as long as you make it now (maybe mix some tylose into the fondant if you have some) and let it dry all week (gumpaste is stronger if you can use that) it will be plenty strong. I like to use skewers sometimes too, if I want them longer than a toothpick to go deeper into the cake.