I’ll settle with Japan giving Germany two “I-boats” (basically the Japanese version of U-boats, but with totally different functions due to differences in doctrine). I call them I-boats because all the ships in this group of ships (with several classes) started with the letter”I” (there was also the shorter range, possible cheaper, older, and generally inferior RO- boats). These submarines carried the type 95 torpedo, superior to the famous type 93 on Japanese cruisers and destroyers. More submarines for the Battle Of The Atlantic is always a great thing, and U-boats, especially those manned by great crews, with these torpedos could have been deadly.
In return, Germany could give Japan some of their experience in radar and optics-it was a real problem for the Axis, not sharing technology with each other, even though the Kriegsmarine was ordered to under direct orders from Hitler (who they didn’t follow). This is especially a problem as the Allies did share technology with each other.
I think German panzers would not have helped Japan anyway, as they would have been too heavy to transport, and armoured warfare in general was not needed on the islands of the Pacific and China (who didn’t have many good tanks at all, so Japan had no need to have better tanks). If I had to choose a land weapon for Japan to use, I would choose the 88mm anti aircraft gun. A superb anti tank gun that can fight any US tank and possibly a good help for the anti aircraft armament of Japan’s ships (if it could be adapted to naval use).