Here's my changes.
1. Added an alpha channel so the base layer has transparency. Layers > Transparency > Add Alpha Channel.
2. I duplicated the layer twice. I set one of the duplicated layers to Screen and I set the other to Overlay (can be done by selecting Mode: Normal in the layers dialog and changing the mode). I adjusted the opacity of each layer to about 30-60% depending on what I thought looked best. Sometimes I duplicate the screen layer twice to make it even more bright.
3. I used the FX-Foundry plugin and under FX-Foundry > Photos > Enhance I ran Eg ISO Noise Reduction and Eg Recover Shadows. I may have also used EZ Improver though I'm a little fuzzy in my memory on that detail. Here's where you can get the plugin from...http://gimpfx-foundry.sourceforge.net/
4. I right clicked on the top layer and selected New From Visible... This created a layer which combined all of the changes I made into a single layer. On this final single layer I performed step 5.
5. I further enhanced the image smoothing using the G'MIC plugin. I used Enhancement > Smooth[anisotropic] and set sharpness to 0.84 and anisotropy to 0.18. The rest of the settings I left default. I didn't know the values to use, I just played with the sliders and decided on what I thought looked best. You can download G'MIC and place it in your plugins folder.http://gmic.sourceforge.net/
Since GIMP is available for so many platforms the plugins appear to have several downloads for the same thing. Just be sure to pay attention because they are separated by platform. You need the G'MIC plugin for Windows if you're using MS Windows.