Yeah green tea is nice, but subtle. Capsules of L-theanine (200mg?), caffeine (60mg) with taurine and vitamin C to flesh it out is a lot less subtle. Having two capsules is stronger yet again.
Mixing with R-Modafinil will drastically change memory function too, though a racemic mix seems stronger yet again. Modafinil is very interesting stuff, and it mixed with vaping is quite enjoyable.
Agmatine sulfate is another interesting compound to utilise, it can negate cannabinoid, and basically any drugs tolerance build up, boosts REM sleep and a plethora of other underexplored interactions.
These are all generally considered safe, even at doses much larger than needed too, but like anything will effect everyone slightly differently, so it may not be suitable, but the chances are in our favour.
There's also Alpha Brain and a bunch of nootropics research and concoctions at
www.onnit.com which I have been meaning to look into