I don't think this is possible without there being a war to stop Hitler's war. It needn't be a preventive war, but the Nazis would have to be stopped because otherwise,they would start awar. That was the plan. But I'll throw out somerandom thoughts here (bear in mind I'm not an expert on the nasty century)
Idea 1: The Anschluss War. Hitler's desire to take over Austria early leads him to antagonise mussolini. Musso decides to turn himself into the protector of Southeastern Europe and makes pronouncements about Austrian independence. The Ständestaat government embraces him as its new daddy (they had just seena domestic Nazi coup attempt), and maybe some other Eastern European copuntries join, too? Anyway, at the end of the day there is a German attempt to steamroll into Vienna against international resistance, it turensinto a bloody merat grinder in the Alps, the French government intervenes, this becomes Hitler's Winter War and though he gets a nominal victory, the cost has completely bankrupted his regimne and he is soon after toppled by a Wehrmacht officer coup led by Goering.
Idea 2: Dick in the chorizo grinder. The Spanish Civil War turns into an even more protracted nastiness as a French left-wing government starts supplying the Republican side. Hitler feels obligated to send moretroops and gear, and eventually has to do it openly. in the process, he antagonises Britain over sea transport of troops thrpough Portugal. London leans on Lisbon, the supply route closes, Britain closes with France inahard anti-German stance. An earlier rearmament means no need for an Appeasement policy. Hitler, having put serious amounts of money, gear and bloodinto Spainm, withdraws and tries other political projects, but meets too much resistance. As his gambles turn increasingly reckless, he has an unfortunate accident. Germany turns into your bog-standard right-wing authoritarian hekllhole until, ten years later, the Mefo debt topples the government, or the Soviets invade.
Idea 3: Successful Socialist/Communist revolutions in more Eastern European states. Give Stalin a third avenue to exploit, instead of dividing the spoils, he turns himself into the benevolent hegemon of Germany's intended victims. Of course they won't come willingly, but just as Hitler can look preferable to Stalin for a bog-standard bourgeois authoritarian or ethnic nationalist, Stalin can look better than Hitler to a run-of-the-mill dictatorial Communist or revolutionary Socialist. War preempted, or at leasta Nazi-initated one (the spectre of a Western-supported anticommunist crusade that turns Hitler and Mussolini into continental swordarms is NASTY).
Or, of course, a cheat:
"...as the Führer exits the opera and descends the stairs to his waiting car. The spectarors to the right and left are saluting, dressed in rich evening wear, the men in dinner jackets. It is true, the Germans do love their charismatic leader, especialy those assembled here an Bayr... [shouting, sound of gunfire] What is now... I am sorry, ladies and gentlemen, there does appear to be something irregular happerning ... guards are forming a cordon. Several people are down and now it appears that Adolf Hitler, Führer and chancellor of Germany has been shot. The limousine is leaving at high speed, you can hear the sirens, and I believe the shooter was heard shouting "Rot Front", the salute of the German Communist pa [transmission ends]