You don't tell us what datasource you are working with.

If your buildings are polygons, you can select the ones you want to see, export them to another shapefile with Save Selection As..., add the result to the canvas and remove the original layer.

Same should work for contour lines.

You don't tell us what datasource you are working with.

If your buildings are polygons, you can select the ones you want to see, export them to another shapefile with Save Selection As..., add the result to the canvas and remove the original layer.

Same should work for contour lines.

UPDATE

The function has now been moved to Save As ..., [x] Save only selected features checkbox.