I think the most realistic transfers of german "stuff" to the japanese are things that you can either cram into a submarine or ship via plane before operation barbarossa starts, so by necessity they would have to be either rather small or something that one would not need to many examples of, so I totally agree with mostly sending blueprints and examples of smaller technological items.
The big areas the germans have over the japanese in technologies I would consider "shippable" are radars as other posters already pointed out, inline- and jet aircraft engines (especially ones with better performance at altitude), certain small arms (SMGs come to mind) and man portable anti-tank weapons (or basically anything involving rocket engines). Larger items like tanks quickly run into problems with the japanese infrastructure and the nature of the war in the pacific: How much use is a Panther or Tiger going to be on some small pacific island and how do you get it there assuming the Japanese even manage to build a reliable example.
The later part is actualy the biggest hurdle overall: Japanese manufacturing techniques and the way their industry was organized already had pretty significant problems with mass producing certain things (especially electronics components) and even more so as the war progressed. In aircraft for example the Japanese were perfectly capable of developing extremly good designs (look at the Ki-84 or J7M) but could not get reliable enough engines manufactured (not so much designed) and had to make do with awfull fuel quality.
So maybe the best way for Germany to help Japan (and Italy for that matter, as they had some of the same problems) would be industrial assistance as early as possible, but Im not sure how feasible that actually is and doubt if the short timeframes allow for any meaningfull change.