I think, as mentioned, your earliest possibility is Eleanor Roosevelt. After helping secure support for Stevenson in '52, Eleanor is chosen as his running mate. The ticket loses but does better than OTL. Some people say that had Stevenson picked a man, they could've beaten Ike. Others believe that Eleanor is what made them competitive against Ike in the first place. The '56 convention is a mess. More candidates enter the fray, not necessarily because they can beat Ike, but because they know Stevenson can't. This includes Kefauver and Harriman per OTL. LBJ also makes a more serious play. There are a few more candidates - take your pick of who. The point is, the convention is deadlocked. For the first time in decades, the balloting goes beyond a third ballot. Delegates are undecided. Finally, a compromise candidate emerges, Eleanor Roosevelt. She goes on to narrowly defeat Ike. Likely? No, of course not. Plausible? Enough.
Next, Shirley Temple wins her Congressional race in 1967. She wins the Republican nomination in the 1976 Senate primary. She becomes close to Reagan throughout his presidency and he encourages her to take the lead on a number of issues - foreign and domestic for his administration. She loses the 1988 presidential primary to Bush, but he chooses her as his Secretary of State. In 1996, she is the Republican nominee and defeats Clinton who may be vulnerable to a conservative woman on the ticket. I am positive there is an earlier way to get her to the White House given a career beginning in 1967, though. This assumes relatively few butterflies after her Congressional victory. It is entirely possible, however, to have Bush win the 1980 nomination and have Reagan encourage him to choose Temple as his running mate. Bush serves from 80-88. Temple, the conservative voice in his administration, is the 88 nominee and is elected the first female president that year.
Ferraro was likely poised to become the first female Speaker of the House after Tip O'Neill. Not only was she rising fast in leadership, O'Neill adored her and had great respect for her. Let's say that Mondale chooses instead to offer the nomination to Feinstein and Ferraro stays in the House. When O'Neill steps down in 87, Ferraro becomes the Speaker. Instead of assuming the compromising role that O'Neill did, she becomes a tough liberal alternative to the Reagan administration and, eventually, the Bush one. As Speaker, Republicans (particularly Gingrich) try and bring her down with the "scandals" that plagued her 88 bid. This allows the country to deal with them and get them out of the way prior to a presidential bid. There's maybe even an ethics investigation that clears her. Either way, by the time 1992 roles around the Democrats are without a candidate. Mario Cuomo hedges and instead encourages Ferraro, a close friend, to run instead. She steps down as Speaker to run and decimates anyone on her way to the nomination. Then, 1992 takes a turn, and she becomes the first female president. There's also the path to the White House she envisioned for herself: winning the Senate in 92, beating Gore in the 2000 primaries, and then winning the general against Bush. Her cancer, however, complicates that.
Another favorite of mine is Elizabeth Dole's path. This relies on Bush's 2000 campaign falling apart. It is probably easier to have him lose against Ann Richards (he never meets Karl Rove or some other list of events or maybe the DUI comes out early and neutralizes his character attacks on Richards). In this case, you could have a 2000 general election that is Richards vs. Elizabeth Dole. Two female nominees and a guaranteed female president. Or, you could have Dole beat Bush. This requires a lot more work. Basically, the scenario I've envisioned is the Bush DUI breaking right before the Ames Straw Poll. Dole wins the Straw Poll - a major electability test at the time. She uses that to raise money and become viable against Bush. It's a drawn-out contest that makes Hillary and Obama look like rainbows and roses. Dole scrapes by and then goes on to defeat Gore with McCain as her running mate.