Lonely Are the Brave and Seven Days in May, but many of them are long forgotten. He did contribute one major thing to American culture--he purchased the rights to Ken Kesey's One Flew Over the Cuckoo's Nest, had it adapted into a play, and took it on stage. By the time he could get the film made he was too old to play the part of McMurphy, and his son Michael produced it with Jack Nicholson.
I think his last very good film was Posse, which he directed (he also directed a forgettable pirate film called Scalawag). Released in 1975, it's a revisionist Western that has a lot of the tropes of the genre--a really great shootout that has some great stunts, and a lot of action on a train--but is really the tale of political ambition and how it can blind someone.
Douglas plays a U.S. Marshall who always gets his man. He is also running for U.S. Senate. As the film begins he is chasing after notorious train robber Bruce Dern. He pays off an informant and finds Dern's gang, killing them all, including burning some alive. Dern escapes, but is finally captured, and Douglas makes political hay out of it. But Dern still has some tricks up his sleeve.
The film is a nice allegory about the political process. Douglas is clearly corrupt--he's bought and paid for by the railroad, but his posse start to wonder about his loyalty. There is also the aspect of the press--James Stacy (who just died last week) is the local journalist who questions Douglas' motives.
If there's anything I can say about Douglas as I move on to other centenarians, it's that he never played it safe. He, like many stars of after the studio system, made his own films with his production company, Bryna (named after this mother) that weren't safe and were provocative. Some were successful, some not. He is largely responsible for the end of the blacklist and launching Stanley Kubrick's career. He never won a competitive Oscar, but was under-rated as an actor. In Posse, we don't get that moment of intense anger until the very end of the film, but it's worth the wait.
Douglas turns 100 in December. Long may he live.