Hey DJ,
JI is right. Increasing the ISO will do absolutely nothing [try it, all the images will look the same, only the amount of noise will change].
Your suggestion of fill in flash is correct. But you must learn how to balance your exposure ratios with the flash. Imagine your image has two main areas you wish to render: 1. the bike and foreground and 2. the sky. Use the flash to correctly expose the bike and foreground [don't forget your exposure compensation] and then use the shutter speed to optimise the exposure of the sky/background.
Its a real balancing act, so have a practice, and then practice some more…
You will have to ensure your colour temp is set correctly as the flash is daylight balanced and the sky in this case is cloudy - normally you would worry about the main image [the bike in this case] and live with a slightly grey sky.
If you are trying to duplicate a normal sunny day exposure, it will take some work. But you can mess around and get some great creative effects. But as always shot in RAW, not jpeg. As you can see from the image I adjusted [it is just a quick and dirty adjustment] you can see that the data has been degraded as the original I grabbed was a low res jpeg. With the RAW image you wouldn't get the breakup in the shadow areas.
Good to see that I was in Taupo at the correct time of year, that just looks cold and wet.
See you soon,
VMX42