Well, basically a blur, Look in Filters -> Blur -> Gaussian Blur.
You could try something more "arty" say an Orton effect (look it up) There are some old scripts for this but this one is interesting http://gimpchat.com/viewtopic.php?f=9&t=21457 download the windows version and follow the installation directions. Once installed, find it in Tools -> GEGL Operations -> Orton (in the drop down list)
Play with the sliders. An example:
You could try something more "arty" say an Orton effect (look it up) There are some old scripts for this but this one is interesting http://gimpchat.com/viewtopic.php?f=9&t=21457 download the windows version and follow the installation directions. Once installed, find it in Tools -> GEGL Operations -> Orton (in the drop down list)
Play with the sliders. An example: