Having sough inspiration from this site on a number of occasions I think it is time to give something back. I have done a write-up of modifications for the Vespa GTS 300 HPE on my own homepage http://stenberg.dk/Vespa300hpe.htm. These mods are focused on getting it ready for summer road/camping trips.
* Side cases (Monokey / GIVI Trekker Dolomiti)
* Spare fuel canister
* GPS "cockpit"
* Side stand foot mod
* Speedometer fix
* Oil pan inspection glass
* Bag hook replacement
My homepage has much description and pictures.
The Fehling rack turned out to be quite a big project. The GPS holder is a bit of a mess to look at - but it does the job. There are mixed opinions on flip-out side stands - but I like this one. I love oil pan inspection window. And I think it is a must to replace the plastic bag hook