Focus you biggest efforts on getting straight A's, scoring high on the GRE, and observing physical therapists in as wide of a variety of inpatient and outpatient settings as possible. Also keep in mind getting to know a couple of PTs and at least one professor well so you can get some strong LORs. Participating in any extracurriculars or volunteer activities at the expense of these things will actually hurt you not help you.
If you can dedicate your time to any "resume builders" without any of the above slipping, I would consider picking one thing and sticking with it. Devoting a good amount of effort to one thing over a long period of time looks better in my mind than dabbling in several things. Lots of little things just look like fluff on an application, and you've already got a few fluffy things to throw on there for filler. PTCAS only gives you like 1 sentence to describe each activity anyway, so the length of time commitment and number of hours have to kind of speak for themselves. At least that's my theory. I don't have any specific evidence or data to back that up.
However, in my (relatively) extensive study of the PT application process, I have not seen strong evidence for Extras/Volunteering/Awards/Honors/etc being a
large factor in the PT admissions process. In other fields, such as med school admissions, these things seem to matter more (thought strong GPA/MCAT is obviously still the biggest factor). With PT, it is a lot easier to make up for lame extras with a killer GPA or GRE than it is to make up for a lame GPA or GRE (or observation hours) with awesome extras. The extras aren't really quantifiable and since a lot of schools decide which students to admit largely based off of numerical rankings, the only way I can see them contributing is if they are somehow externally scored, or if they contribute to your interview score in some way. Again, this is just my theory based on what I've observed. Hypothetically I would think that extras are going to matter less at schools that get >1000 applications, because they just don't have the manpower to qualitatively analyze everyone's app; perhaps at less "popular" schools that only get a few hundred applications they could matter more? Who knows.
These are just a few things to bear in mind as you decide what to get involved in. And as
@Watson27 said, if it is something that you would do anyway, even if you weren't applying to PT school, then by all means have a blast. If it's something you are going to dedicate 5 hours a week to for 1 semester just to "boost your application", it's probably higher yield to spend those hours observing an additional PT setting, studying for the GRE or studying for your classes (or getting a job and making some $$).
And of the things you have mentioned, getting involved in some kind of academic research is probably going to the be the biggest application strengthener. And I totally agree with
@futuredpt01 that the biggest impact you extras will have is if they happen to come up during an interview and you are able to expound on them beyond the lame description that you are constrained to on PTCAS.
Hope that helps. Let us know if you have other questions.
