Do senior (on the job longer) police officers get the "cool" assignments, like standing watch at a concert/playoff game/Super Bowl?

depends on the local government. in some, union rules dictate who gets first crack at such added work and that usually means seniority rules -- of course, officers scheduled to be on shift anyway aren't eligible

Most of that depends on department policy and/or union agreement, so it varies.

Concerts and sporting events are almost always overtime shifts, as they can't pull that many officers from their normal patrol. On my department, they divide overtime as equally as possible so first choice goes to the person with the least overtime for that year. That is uncommon, though. Other departments in my area let the most senior person get first choice.