The cemetery is worth a quiet, respectful visit, whenever school is NOT in session.
Other than that, Arlington is just alot of headquarters of national groups, strip malls, etc. For the quaint colonial feel, go to Alexandria. For a quick lunch at a chain restaurant on a metro stop, go the Arlington/Pentagon City/Crystal City route.
My mom lived in Arlington for eight years, so I spent way too much time there. It’s a neighborhood/workinghood type place, with normal life going on and nothing extraordinary about it (outside of the cemetery). Not somewhere I would waste time in when I could be touristing in DC.