I live in DC (in the city)
Cactus Cantina is on Wisconsin Avenue and is not reasonably walkable to the metro. You could get off the red line at Van Ness and take a taxi for around $10 to Cactus Cantina.
Unless you Love Love zoos, I would skip the National Zoo. To me, it's just another zoo like other zoos in major cities. It's a major, major time eater (a whole day) and like a PP said it will be PACKED on Easter Monday. Like, Disney on New Years' Eve packed.
I would concentrate your visit on the Smithsonians, Arlington National Cemetary and the Capitol. Also, be prepared for long, long, long waits to get into the Egg Roll. It is a very long day, with bag searches etc. The kids activites, while well organized, have very long waits associated with them once you make it onto the White House grounds. Be sure and eat/drink before you go, unless it has changed this year there are no real refreshments available.
Let me know if you have any more questions be glad to answer if I could!