A decent compass might be an idea....Silva preferably (you may get individual issue these days? ....don't know)
Packets of plasters (can be bought on camp anyway..)
Tube of Algipan and length of tubigrip (arriving with your own saves going to the Med Centre for any ailment ...issued by the medics for anything from minor calf strain to toothache..)
I would have suggested a small tin of Duraglit..in readiness for your sweatnights and block-cleaning marathons ..... (bringing the copper pipes in the bogs to a bright shine ...aah..memories...and SO satisfying!!) ......but you probably don't do THEM either these days !......... Assuming there's still 'brass bits' on yer No2 Dress uniform then having a secret stash of Duraglit doesn't harm .... (Don't let every other feker have it off you either)
Packet of 'Wet Wipes'
HEADTORCH!!! unbelievably useful timesaver ..saves going into 'stick torch in gob and try point it' mode while yer scrabbling around in the bottom of your rucksack in the pitch dark....(..and a few spare batteries..)
Oh.......and see if you can get a spare set of keys to the parade square (try eBay..) ....yer Cpl will be impressed if you turn up with your own set...

...........and in honour of Bruce Forsyth's 80th birthday....what about "a set of wine glasses and a cuddly toy..." (Didn't he do WELL!! ) ...drum roll..cymbals...crash