As you mentioned, your career interests may change, and at the moment seem to be rather varied. However, there are a few things you can do to prepare for a graduate career.
1) Research the programs you are interested in (Harvard, Stanford, MIT) as well as similar programs that you would be willing to apply to. You will likely need to split this into two groups: architecture and law. Once you've identified approximately a 6-12 of each type of program, look at their admissions criteria, recommended coursework, and suggested extracurricular or work experience. Most graduate programs will indicate the grade point averages, standardized test scores, and subject matter experience they desire in applicants. Generate lists of the most frequently mentioned criteria.
2) Examine the offerings at Hamilton College in relation to those lists. If the law schools all request you take Intro to PolySci, be sure to work that, or an equivalent course, into your schedule. If the architecture schools all require you to submit a drawing sample, plan on taking at least one art class so that you can develop a portfolio (and possibly a letter from the professor). If certain specialty courses are unavailable at Hamilton, see if you are able to take the class at a nearby university, or complete it as an independent study.
3) As you narrow your career interests, use the summers (or possibly the school year) to engage in internships or jobs in those fields. Entry level work in a specific field and internships are often seen as markers that students are driven, organized, and mature. You may also consider joining undergraduate student organizations in these areas, such as the American Institute of Architecture Students (https://www.aias.org/website/article.asp?id=8), which can engage you in the field at a national level.
4) Regardless of where you apply, you will eventually need letters of recommendation. Be conscious of this while in school and develop strong relationships with your professors. Speak up in class, volunteer for projects, and seek their advice on your career.
I attended a small, private liberal arts college and used similar steps to prepare myself for possible graduate careers in biology and psychology, and found them to be effective. Although the school may not have been a tier 1 research institution or offer classes in every subject, the flexibility to complete independent projects and faculty support were invaluable.