Easter is problematic. Most restaurants I'd suggest reservations, as it is chaos with families doing the same. We have outdoor dining during specific periods, which I recall begins on April 1st (could be wrong). You are going to be hit or miss where that is concerned.
There is also a restaurant on the Bike Trail just beyond National Airport is the Washington Sailing Marina. They have a restaurant across from the run way (noisey) and Potomac. I've just searched for the name (which escapes me), and cannot find on google or TA for some reason...so is possible it's closed.
My advice is as follows....if you are biking and wish for a view...Reserve at Chart House they have the best view. Also consider reserving at Mai Tai on King ask for a seat upstairs when you reserve. If your plans don't accommodate a specific time (to reserve a table)...go to MoMo Sushi (Queen Street) or any restaurant...order your meal to go...walk to the waterfront and have a marvelous picnic in one of our two waterfront parks that line the Potomac!