Czechoslovakia is surprisingly easy military-wise; you basically cannot be invaded via the Sudentland, so there are really only two threats to worry about: an invasion via southern Poland, once Poland has fallen, and an Axis Hungary marching into Bratislava; the best solution for Hungary is pre-emption, if you can DoW them before they join the Axis then they should be no match, alone, for Czech troops. Poland is more delicate; you can try to prop up the Poles, but even the Sudetenland forts won't stop the Huns if they're unmanned, so you have only limited forces with which to intervene in the open Polish plain, where they risk being overhwelmed and destroyed by the Germans. One tactic is to join the allies after the Germans attack Poland, and overun Austria and southern Germany when the AI has them unprotected; it won't cover your weak flank but it will fuck up Germnay and provide a nice boost to your industries and ressources. The one problem with the Czechs, of course, if that you don't have enough ressources to run your industries.
Turkey is another good option. Both Persia and oil-rich Iraq are pushovers, although DoWing them will be a pain; non-democratic minors are probably the better choice because you can DoW more easily and strengthen yourself before the world war begins.