As I mentioned in my post earlier in the week about riding the elevator to the top of the Washington Monument, we got downtown just past 9 a.m. so parking was easy. We parked down by the monument, however, it looked like there was plenty of parking along Constitution Avenue (previous experience has shown me otherwise. The key is to get downtown early if you want the free weekend parking - weekdays do have limits - I think its 3 hours).
The Smithsonian Museums open at 10 a.m. and I found the first hour is often a good time to visit, as the museums are generally quiet. We went straight to the Butterfly Pavilion before seeing some of the other exhibits. We got our tickets, which were 5 minutes past time of purchase. There was a short line, but it moved quickly.
As always, the butterfly sightings are spectacular. You can really get up close to them (sometimes they even land on you - one attached to me on this visit, but it was behind me so I couldn't see).
Being my visit was on a Sunday, there was an entry fee (the admission the museum is always free, but some of the Smithsonian's exhibits do have a fee, this is one of them). However, if you want to go to the Butterfly Pavilion on a Tuesday, it's free. Just keep in mind, it might be tough to get tickets for the time frame you want on Tuesdays. Probably best to get your tickets upon arrival at the museum.
It's a fun experience!