IOTL, Bulgaria and Czechoslovakia could have been kept free of the Soviets except for mistakes made by their own governments.
Bulgaria had never declared war on the Soviet Union, and by summer 1944 was negotiating a peace with the British and Americans. That dragged on too long because of their refusal to declare war on Germany as price for peace with Allies. Instead, the Soviets declared war on Bulgaria, invaded the country, and set up a "popular front" government that allowed them to be subverted. If Bulgaria had moved faster to seal peace with the Allies, US or British paratroopers could have entered Bulgaria before the Red Army arrived at its borders, and Stalin would not have risked invading the country.
Czechoslovakia was actually free of Red Army troops when it fell to the Communists in 1948. If Benes and the non-Communists in government had acted in concert better, they could have avoided the Prague Coup and aligned itself with the West in 1948.
So those two countries are the easiest to handle. No battlefield PODs are really needed.
Romania and Hungary are harder. Both had declared war against the Soviet Union and were farther from the West. Both would require either a substantial delay in the Soviet advance, or a spectacular Allied advance through Italy. Probably both. In that regard, Hungary is easier to reach. However, if it can be, both countries were willing to betray the Nazis and ally with the West. If we assume the Soviets are delayed in the Ukraine, and the Allies can reach Hungary through the Ljublana Gap, then both Admiral Horthy and King Michael could achieve what they tried IOTL which is to defect to the Allies soon enough to prevent their countries occupation by the Soviets. In such a case, it is even possible that being isolated from Moscow, Tito is forced to play nice with the Yugoslav Royalists and may even be removed from power later on. But we'd need the Soviets to be delayed by about a year. With an early enough POD on the Eastern Front, it's possible.
The hard part is Poland as Stalin needs control of Poland to maintain his supply line to any forces stationed in Germany, and the Central European Plain is the obvious invasion route into Germany.
The only way to get a Poland free of Soviet influence is if the Germans collapse in the West while still fighting the Soviets in Belarus and the Baltics. At that point, the Polish government in exile takes control of Poland with Western support.
A more likely scenario is one where the Poles succeed in a Warsaw Uprising situation and complicates postwar Poland where the Soviets also have a lot of influence.
Germany is going to be divided into occupation zones regardless. After WWII ended, nobody intended to create two separate Germanies or start a Cold War. They were just trying to think of how to prevent Germany from starting a third war. How Stalin reacts to events will determine the eventual fate of its occupation zone. If the rest of Central Europe is free of Soviet influence, I don't see him pushing hard to create an isolated East German state, but coming to some other terms.
As for what can be done to delay the Red Army's advance by a year or more, there can be very PODs. I'd prefer one where Hitler realized he was overextended in both late 1941 and 1942, and the German Army is hunkered down and prepared when the Red Army launches their Winter Offensives. With less German and more Soviet casualties, Germany will be able to hold out longer.