How can I add editable content boxes to my site?

What is the best way to do this?

I’d like to add a bunch of editable content boxes to my theme. Mostly I’ll be using them for text. I’d like to put PHP codes or shortcodes into the script in order to call the boxes. I’d like the content of each box to be editable in the admin section.

Widgets don’t exactly do it because the admin is able to change the number of widgets and the location. They’re also a bit cumbersome to create and manage.

Thanks!
M