Help! I am going into my senior year of high school, and I feel completely unprepared for college applications. I have always planned on applying to top-tier and Ivy league schools, but as I have started finalizing my college list and getting started on applications, I feel like I have no chance of getting into those schools. My grades and test scores are fine, but I have nothing that will stand out to top colleges. I am trying to plan what to do in the final few months before my applications are due, but I feel like there isn’t much time, and I don’t want to look uncommitted or overzealous in my college applications if I just all of a sudden start a whole bunch of new activities my senior year. What can I do to improve my applications?
Choose realistic reach, match, and safety schools. Write thoughtful essays. Get a summer job. The time for new ECs has passed.
For a first approximation, compare you numbers with those of colleges’ Common Data Set to see where you stand.
If there were things people could do 4 months before college applications are submitted to boost their chances at top colleges then everyone would do them. That should suggest that the record you’ll have when you apply is the record you have built up to now.
Except for your essays, LoRs and maybe your summer activity your college portfolio is pretty much done at the end of Junior year. Now it’s time to tie all of the random pieces into a narrative arc: who you are and where you are going. Use your essays (and help your rec writers) to connect the dots.
And, figure out what (besides the name on the sweatshirt) matters to you about college. You list Cornell, Columbia, UChic, Stanford and CMU on your other post: why those? Figure out what the specific draw of each is for you, and then consider how you fit what they are looking for. Then go put your back into finding a list of safeties and matches that you would genuinely be happy to attend.
If prestige is a big driver for you, be honest with yourself about that - and look for ways to get enough of a sense of prestige from your safeties and matches. You will find something to feel proud of in the most amazing places- world-leading profs turn up in universities of every level, and many colleges that seem otherwise unremarkable will have a department or centre that is genuinely a stand out -to the cognoscenti 
choose at least 2-3 safety schools so you have an option out of those if you don’t get into your top choices. Transferring in 2 years is also an option if you feel like you wanna give another try to getting into your top choice (in the worst case scenario that you don’t get in)
…be aware that “the worst case scenario” is also the most likely scenario for the colleges you have listed so far…