Use the remilitarization of the Rhineland and the non-existent Western response to it as the pretext for the diplomatic realignment I'm going to undertake.
Offer recognition of their possession of Zaolzie to the Czechs in exchange for two things: an alliance against potential German aggression, and greater military and industrial coordination to prepare for that event.
Forge closer ties with the Romanians as well.
Forge closer ties with Moscow. Keep enough of a distance to not be blacklisted by the Western powers, but privately make it clear that Poland is not and will not be an enemy to the Soviet Union in the future. Also sell the idea to Stalin of making Poland a dumping ground for Soviet dissidents and people he would otherwise have purged; the real prize here would be getting, say, Tukhachevsky to run my armored divisions when the Great Officer Purge begins.
Invest in a better transportation network. Primarily railways, but the road net as well. To as great an extent as possible, decentralize the system to make it harder to cripple it simply by taking out one junction point. Increased telephone and radio penetration would also be useful.
Improve agricultural production by organizing and incentivizing farming cooperatives. Allowing farmers to pool their resources a la the grange movement in the US would help immensely, and would also provide a good excuse for the militarily-useful decentralized rail and telephone net.
Similarly, establish a number of regional airports throughout the country. In the event of war, these can all be rapidly converted to military airfields. Emphasize reliability and ruggedness in aircraft development; ground crew training and a proper ground control network are also essential to the task of giving the air force the information and rapid turn-around time essential to surviving the German onslaught. Priorities are to develop an interceptor, a fighter-bomber, and a reasonably accurate medium bomber. The interceptor is meant to obtain air superiority, the fighter-bomber can either repel enemy aircraft or strafe enemy troops (and will be built with anti-tank grade machine cannon for that purpose), and the medium bomber would hit enemy concentrations, airfields, and supply dumps.
The navy gets two rowboats and a rubber ducky.
The army itself is to be motorized to the greatest degree possible. In particular, the artillery and logistics train must be motorized. The basic philosophy around which the army is to be designed is mobility; static defense is implausible at best given the geography of Poland, especially in relation to the most likely German attack vectors. The army must be well-trained and well-equipped; the necessary reduction in size to be able to afford such only reinforces the need to keep the troops mobile. Development of a capable dual-purpose anti-tank and anti-air gun is paramount; furthermore, use of the gun in this manner is to be explicitly included in training of its operators.
Part of the reason for ties with Romania (and the Soviet Union, for that matter) will be to ensure a steady flow of oil to keep the fuel situation in hand. Refining, however, will be done in Poland itself. Aside from the immediate value of having a local source of gasoline for the military, and the secondary value of being able to sell refined fuel back to the Soviet Union, having a refinery will encourage the development of local industry that uses oil byproducts. Artificial rubber would be of particular value, but many other useful possibilities exist.