The boulder that blocks Romani Ranch will get removed at the dawn of the final day if you don't blow it up with a powder keg but playing the Song of Time when it's gone will always bring it back.
If you talk to Romani any time before 6:00 pm on the first day, she will give you the balloon shooting challenge and if you win it, she will teach you Epona's Song and remove the gate that blocks Epona and that gate will stay gone even after you play the Song of Time or if you already learned Epona's Song, she will give you nothing. The aliens come at 2:30 am whether or not you're in Romani Ranch and the game keeps track of their movement even when you're not there. You can actually skip the cutscene of them coming but still do the alien fight by entering Romani Ranch after 2:30 am but before they make it to the barn. When you win the fight, Romani will give you a bottle of Milk, or if she already gave it to you before, absolutely nothing. All of them no matter how far away from the barn they are move at the speed that makes them take 80 minutes of game time to get to the barn. Every time you shoot an alien, it reappears right away where it started. Letting them get to the barn before 5:15 when they normally leave counts as failing unless you already won the fight before since you last played the Song of Time. Not being in Romani Ranch at 5:15 am to see the cutscene of them leaving also counts as failing even if they didn't make it to the barn by then. Once you fail, the roof of the barn is sucked off and there are no longer any cows in it. There's actually a hole under the missing roof where you can hover up to then fall through to void warp. If you never won the alien fight since you last played the Song of Time, no matter how many times you fail the alien fight on the first day then reset the first day, the roof and the cows will come back when you reset it and it will be as though you never did the alien fight yet. If you reset the first day after you won the alien fight, the aliens will appear again at 2:30 am but won't start moving until 4:10 am so they won't have enough time to make it to the barn by 5:15 am so you'll automatically win the fight just by being there at 5:15 am. Not being in Romani Ranch by 5:15 am to observe the cutscene of winning the fight will still count as winning the fight because you already won it before since you last played the Song of Time and the roof will stay on. Once the roof is off and the cows are gone, there's no other way to make them come back than resetting the first day and playing the Song of Time.
Once you won the fight, you can ride with Cremia any time from 6:00 pm to 7:00 pm on the second day no matter how many times you reset the second day as long as you never rode with her or were there at 7:00 pm to see the chariot leave since you last played the Song of Time. You can also skip the cutscene of riding out of Romani Ranch with her by doing the Day Reset glitch in Romani Ranch and then activate the bandit fight using some unknown trick. Also, the fences in Milk Road are rearranged when and only when it's the night of the second day and you already won the alien fight since you last played the Song of Time. Once you either ride with her or are in Romani Ranch at 7:00 pm to see the chariot leave, you will never get another chance to ride with her until you play the Song of Time. If you stop all 3 Milk bottles from getting broken, you will succeed and Cremia will give you Romani's Mask just outside the east entrance of Clock Town, or if you already have it, a huge rupee, or if you already have a full wallet, a hug, and as soon as the cutscene ends, it will be 9:00 pm of the second day. If all 3 Milk bottles get broken, you will fail and get nothing and return to just outside the barn and by the time the cutscene ends, it will be 8:00 pm of the second day. Each Milk bottle takes 3 hits to get broken. The Milk bar always sells Milk and Chateau Romani during the time it's open for business even if you fail the milk delivery.