Faced with this problem, I hit the web and found a tool that's fast, easy to use, and gave me exactly what I needed: Floorplanner. While it's made primarily to mock up homes, the tools provided can easily be applied to galleries or event spaces. Your design can have multiple floors, and you can model outdoor areas as well. In a 2D plan view, you drag and drop in walls, doors, furniture, and fixtures. You can display the dimensions of any element and resize elements quickly. And then you can look at your plan in 3D, spin it around and impress the neighbors. You can even print out your design as a 3D model if you want a paperweight version of your future museum.
Floorplanner is free for a single multi-story design, $29 per year for five, or you can check out the Pro accounts (marketed to real estate agents), which start at $29 per month.
The Museum of Latin American Art in Los Angeles uses Floorplanner to produce layouts for special events, using the floorplans both in planning with the clients and as a guide for the setup crew. I'm using it to draft potential layouts for the cafe I'm working on. What might you use it for?