Anyone have any suggestions for easy and inexpensive mods? I have done a lot of sticker mods and a Siamese cube and am wondering what the next step would be.

Truncation and adding to puzzles for shape mods would be a good next step. You can fill holes with milliput or cap them with plastic sheeting. Personally my first was a master cuboctahedron, but you can really make anything you want as long as it doesn't interfere with the mechanism.

Jack

Maybe a cuboid puzzle. The 3x3x5 is pretty easy to build. All you need is a super 3x3x3, an 18 extra cubies (I used a 3x3x4). Its just cut and paste

What CB said, or an Octagon Barrel is another easy puzzle to make.

Start by one of the easiest : an half truncated cube or a square 2.
After, you could try cuboctahedron or master cuboctahedron.
Ater, we can start cuboids : 3x3x5 or 2x2x1