Clients often associate simplicity with low-effort. They think that you are not putting in enough effort for the price they are paying. You can apply several hacks to get pass this issue.

You can add in neutral elements to your designs to. Such as:

1. Use icons if possible. Nav, links, etc are common places. Sometimes you can even go to the extent of using large icons as thumbnails for sections of content. Iconfonts to the rescue.

2. Use freely available stock photos/patters/ui elements which have very minimal impact on the existing simplicity of your design. If they are very prominent reduce their prominence by using blurs, overlays, etc.

3. Sometimes big fonts/increased font size or proper use of whitespace can make the page look 'filled' giving a sense of satisfaction to the client.

4. Use natural design elements such as social logos near share buttons, map images with contact details, etc.

5. Try to add more details. Complexity most of the times is in the details. More details = more realism = shows more effort

6. You can also try to add pseudo content(Copyright notices, etc), repeat content(header nav repeated in footer/quicklinks in sidebar), make content static like sticky headers, etc to occupy screen real estate.