Seconding the point about carriers. If you're staying in urban/suburban areas, go with the cheapest option that meets your needs, they will all work great, including in underground Metro stations. If you're heading out to explore rural or wilderness areas, probably go with Verizon if you need coverage.

Also, shoehorning this in here as a Baltimore resident, yes the National Aquarium is fantastic. Go during work/school hours to avoid crowding. You can take the MARC train from Washington Union station to Camden Yards for $9, and it's about a 15-minute walk from there. Just know that MARC is commuter-focused and doesn't run much in the middle of the day, so make sure to factor that limited schedule into your plans.