I added a Nuprol Viper suppressor - super cheap and I like the look. The internals are a bit strange. A spring and two bits of foam that aren't exactly a good fit. But for a tenner I'm not complaining.
I also added a Red/Green dot sight from an eBay seller. £25 so wasn't expecting much. But fits the job nicely. I can even run it with my iron sights and out of the box didn't need zeroing The batteries it came with were dead when I can back to it an hour later, but I'm sure I powered it off. Good job I have a stock of the tiny AG3's it uses. It has various patterns to choose from, but no brightness setting.
http://www.ebay.co.uk/itm/Quick-release-red-green-dot-reflex-sight-Weaver-rail-holographic-sight/162069768420?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
The seller has lots of different styles for £20 and up. I went quick release so that took it to £25. My brother got a slightly larger fixed version for £20.