A responsive web design is the way to go, I think. I just finished an app using RWD techniques and I am really happy with it. I'm using Zurb Foundation for it. So, holding the tablet device sideways, you'll see a list of categories on the left and their detail pages on the right. But if you rotate the device (or view it in a smartphone), the list disappears and the detail fills the page. The list of items becomes a drop-down at top.
The trick for you will be to show and hide items at smaller and larger sizes. For instance, a smartphone user won't need all those tiny icons showing all over the place, nor column after column of data, but all that can be comfortably viewed on a tablet device on its side.