Is there anything I could do as an undergrad to help improve my department? It's hard to say exactly what would make my department "better", but to be somewhat vague, I would consider research productivity and funding to be important factors to improve on.
There's not much you can do as an undergraduate to help improve research productivity or funding. It's not clear whether the department wants to improve in these ways, or should want to: there's nothing wrong with focusing on teaching. Even if the faculty do want to become more research active, or the administration wants them to, it's not easy and there's little you can do to make it easier. I'd be careful with how you frame things, since you could easily cause hurt feelings by giving your professors the impression that you consider them substandard.
Instead of focusing on what might improve the department in some abstract sense, I'd focus on what would create a stimulating, exciting environment for the current students. You don't want to be too demanding, and some things just might not be feasible, but it can't hurt to ask what could be possible and what it would take to make it happen. You could look into various sorts of activities:
Advanced courses or independent study.
Undergraduate research supervised by faculty.
Informal mathematical activities: math club, Mathematical Association of America student chapter, Putnam team, etc.
If all of these things are regularly available, then your department is doing pretty well. Otherwise, you could try to initiate or reinvigorate some of them. You aren't likely to get clubs or courses for just one student, so you'll probably have to gather some other enthusiastic students. This may be the biggest way you could contribute, as a catalyst to bring together a group of students the department hadn't realized wanted or needed these activities.