Excel is a full featured vector graphics app. It has bundled many shapes and styles, and even supports anchoring connectors to the shapes. All these features make Excel also a good diagram maker.
One of my favorite tips is grid snapping, it can save a lot of time to align shapes. I also recommend resizing the grid as small squares.
I have recorded a screencast for it. The video has demonstrated:
- Resize the grid into small squares.
- Insert a shape to open the Shape Format menu.
- Enable snapping features in the Align menu.
- Insert more shapes and add connectors.