We have had over 70 years to study the events leading up to and through Germany in the 20-40s. With all of that knowledge and these decades of analysis, what could someone done to stop the Nazi party from taking power?
Howdy! FWIW I wrote my Master's Thesis on social movements in the Weimar Republic.
On to your question: Any answer to your question is just as contingent as the historian answering your question as it is on any primary sources that we have. What I mean by this is that the answer is necessarily going to be mediated by how they think social movements and individuals can affect change in their societal surroundings. Some more pessimistic takes might be "there's little that ordinary people can do to shape political events around them because elites control everything" (this would be something from the tradition of Ranke) or "political events occur as the result of near-random chance so it's out of our hands" ( a la John Bury). Alternatively other historians might respond with "ordinary people have immense control over their circumstances and the outcomes of political events are the result of power struggles between individuals/groups." Personally, I fall more in line with the latter view with a dash of chance thrown into everything, but it is worth keeping in mind that there are differing views as to how societal change works.
With that said, many people in the Weimar Republic across all social strata saw the Nazi party as a clear and present danger even as early as the failed Putsch of 1923. One pacifist activist I studied, Constanze Hallgarten, was marked for death as a part of the Putsch and it gave her flashbacks to the brutal counterrevolution in Munich in which she was also marked for death. In this light, she saw them as a continuation of imperial warmongering. She spent the subsequent years of the Weimar Republic organizing peace demonstrations and promoting pacifist causes both in Germany and abroad. One organization that she was a leader of had hundreds of thousands of members and another organization that she founded in October 1932 was able to get over twenty thousand members in just six months- mind you this was only a few months before the NSDAP took power so there was clearly still public sympathy with peace (and anti-Nazi) causes.
It's hard to assess why all the pacifist activism "failed" ( if you think of failure as failing to keep the Nazis out of power). Largely it involves trying to prove a negative, but some of the shortcomings of pacifist activism is that many of the donors to these organizations and the parties they aligned with were quite bourgeois and had a vested interest in not making radical changes to the status quo that the KPD or USPD (which were also nominally pacifist) would demand. Even though the activists were capital critical, they largely believed that militance and warlike tendencies were the result of bad faith politicians and businessmen rather than systemic failings like the far-left wing parties would argue. These differences limited the pool of potential collaborators and also constrained the toolkit of the activists. Additionally had they been farther left, then the bourgeois parties would have nothing to do with them so it was a double bind that was something of a legacy from the split between the SPD and USPD.
On a more individual level, there were isolated lawyers who made it their MO in the twilight years of the Weimar Republic to be as big of a thorn in the side of the NSDAP as possible. One lawyer, Max Hirschberg, took Hitler himself to court over claims that the Nazis were being funded by the Italian Fascist party. Hirschberg ultimately lost the case and many others he led against the Nazis partially as a result of the merits of his cases, but also because of a baked-in conservatism on the part of the Weimar judiciary that was far more sympathetic to reactionary and fascist causes than they were to left-wing causes. That said, lawyers like Hirschberg were few and far between though he could be considered average in that he was just a criminal defense lawyer in Munich and not an active member in party politics or a politician himself.
Needless to say, but folks who actively opposed the NSDAP at any level- be they average Joes or high-level politicians- were among the first to be targeted after the seizure of power and many of these opponents were either arrested or forced into exile.