This is what I planned for my 65th birthday and it worked out pretty well: Get everyone together for an early family dinner then go over to Union Station and get tickets to Washington by night on a sightseeing bus for everyone.
It lasts a couple of hours and stops twice to get out and see stuff up close(#1-Roosevelt or MLK areas adjacent, #2 Lincoln Memorial) and the drives around the monuments all lit up are magical. The drivers are very knowledgeable; buses have huge windows and are roomy for lots of picture-taking opportunities. Easy on the elders legs. Incredibly meaningful to the out-of-towners who only see the Washington Monument or Capitol or White House on the TV. And this will be something they will remember for the rest of their lives.
I can't say how this compares to the sightseeing boat, as I've never done it. I DID do the duck boats and it was fine, not terrific, though, and the night tour blows them away. I went in October, too, and the weather is still fine. The price would be much more reasonable than the boat.
Highly recommend. Fun for all!