What are considered "strong" extra-curriculars?
Law students are typically active, "type-A" individuals and, as a result, are involved in many extra-curriculars that afford them the opportunity to use and improve their skills. This means that most law school applicants have substantial extra-curricular repertoires, making the baseline for "strong" extra-curriculars much higher. Typical extra-curriculars of law students include: club executive; member of a student union/council board or committee; member of a university committee or the university senate; writer for a university newspaper; intramural or varsity sports; and volunteering with local, provincial, or national organizations. Anything beyond this may be considered a "strong" extra-curricular, but bear in mind that deciding whether an extra-curricular is strong is highly subjective and therefore would be difficult to measure with any certainty. The best bet is to focus on doing what you enjoy, rather than attempting to build a resume tailored to "impress" an admissions committee member.
What kind of extra-curriculars would offset low grades or LSAT?
Generally speaking, nearly no extra-curricular activity can offset any grade or LSAT score that would normally preclude you from getting admitted to law school. The largest factor in receiving admission to law school is strong academic ability, as demonstrated by a high GPA, and a competitive score on the LSAT. There are few, if any, substitutes for this, barring exceptional circumstances, such as illness or other significant life event.
What kinds of clubs or activities should I be involved in during undergrad to be looked at favourably?
Generally, demonstrating that you are a "real person", by engaging in activities that are important to you or that you are passionate about, is preferable to being involved in clubs or organizations that you think will give you a leg up over other law school applicants by virtue of the name or position of the organization alone. In other words, don't become president of the pre-law club because you believe law schools will be impressed by the fact that you are the president of the pre-law club. Instead, only become president if you passionate about the club and want to help others on their law school application journey. Usually, the former is very transparent to members of admissions committees. Generally, no particular kind of activity, club, or other extra-curricular is seen as particularly favourable to your application.
Would schools care if I did pre-law courses, was in a pre-law club, or did undergrad moots?
Not usually. You should only take pre-law courses if you are generally interested in the subject matter, as it is unlikely to help convince an admissions committee if your application is otherwise unimpressive. And, if pre-law courses are not your forte, the poor grades you get in them may end up hurting your application. The same goes for being involved in pre-law clubs or undergrad moots.
I worked at a law office and thus have some legal experience. Will this help me?
Probably not. Law schools are generally not interested if you would be a good lawyer; rather, they are interested in your ability to engage in legal academics. While there is a practical skills component of most law schools' JD programs, it is incidental to the main priority, by virtue of the fact that law schools are first and foremost university faculties: critical thinking skills, research and writing ability, and ability to engage with the material. These aspects are best demonstrated by strong undergraduate grades, or in some cases, a graduate degree.