I would first note that it wasn't the Czechs who caved, it was the Entente. The Czechs just decided that when your allies have categorically refused to stand up for your independence, being in one piece is better than being Doomed Moral Victor.
So, is the question what would have happened if France and Britain has stood by the Czechs as a result of German brashness, which was not so far from happening? Well, Germany would lose that war, and possibly the generals would step in to prevent it being fought.
If the Czechs had decided on a blaze of glory, though... as has been said, their armed forced were remarkably throng relative to Germany. I also understand that they had basically quelled Sudeten German agitation by mobilising the army. The population would cause trouble, but the Czechs still have the mountain passes and fortifications that the Germans were so loathe to storm. So certainly the Czechs can hold out for several weeks, inflicting terrible casualties on the Germans.
The question then arises of when France would decide to go back on their decision and intervene, about which I'm not sure.
I wouldn't rely too much on the Poles. Whereas I doubt they'd actually commit their forces to fighting the Czechs, you have to remember that Poland becoming Germany's target was a result of the complete 180 in German-Soviet relations that took place in 1939. Before that, Germany was hostile to the Soviets, Poland was hostile to the Soviets, and although neither party liked the other they talked on ordinary terms. It's conspicuous how Danzig and the Germans in Poland appeared from the mists in 1939 after having gone unmentioned by Goebbels for years. Britain certainly wrote off Poland as a German client. So why should they assume they're next?
In fact, is everybody involved was really, really stupid - a contingency not unprecedented in human history - you could probably get German and Poles against Czechs and Soviets. Unlikely, but interesting.