If you need to simplify your Inventor models (part or assembly files) further than Shrinkwrap can, you need to try this tool available on Autodesk Labs:
This tool is available for Inventor 2013 and only until July 1st 2014.
Simplify the assembly by Including/Excluding components, Define components or subassemblies as a full or partial envelope, and Combine Components into a Simplified Part.
This allows you to filter the Included or Excluded components in the graphics window.
When you click OK, a Simplification Design View Rep is automatically created.
Choose Entire or Partial Calculation to create a Bounding Box or Cylinder of a part or subassembly.
You can then resize the Bounding Box by dragging the face(s) of the Box or Cylinder. An arrow will appear indicating it's being resized.
Edit View Rep:
Make components Visible or Hidden in the Simplification Design View Rep.Combine Components:
Create a Simplified Part and Cap Holes.
Once you create a Simplified Part using Combine Components, you can manually remove all or some of the internal voids on parts/assemblies by capping hole.
Cap Holes is also available in any part file.
Use All to Cap all the Holes:
Use Range that can be used with Detect and enter in a Min-Max hole size value.
For more information on how to simplify models, watch these Autodesk videos: