There are a couple of kinds of mixtures - for the 4th or 5th grade I would suggest you stick to heterogeneous mixtures, which are the type where you can visually identify the bits that are the components of the mixture. You probably want to go with a fairly straightforward separation based on physical properties such as density, solubility, magnetism, etc.
A common separation lab is to make up a mix of two or more of the following: Sand, iron filings, salt, paraffin wax shavings, black pepper, or poppy seeds. Provide a magnifying glass, several containers, water, coffee filters, tweezers, and a magnet, and see if students can separate the components of the mixture.
What really makes this lab a learning experience, however, is the planning process. Asking students to create a flowchart or plan before they begin trying to separate the mixture is essential.
For ideas on formatting the lab and creating good questions, check out the pdf's at this link.