"The Children of Hurin" and "The Fall of Gondolin" are expanded versions of shorter stories within the Silmarillion. I would definitely recommend "Hurin," but "Gondolin" is a little disjointed because Tolkien wrote several versions of different length and style, and Christopher wasn't able to make them into a unified whole before he died.

Also, if you find the Silmarillion to be a slog (the early chapters definitely are), there's a read-along series on the Tor website that clarifies a lot of things. (Tons of spoilers in those articles, obviously.)