According to the Merriam-Webster Dictionary, the primary meaning of the word "abroad" is "beyond the boundaries of one's country; in or to a foreign country."
Throughout his life, Scottish-born author Robert Louis Stevenson was an avid traveler, making extensive and prolonged journeys to France, the United States, the South Sea Islands, and other locations. The poems in question certainly both present the idea and theme of "abroad," but in different ways.
In "Travel," the poet daydreams of journeying to faraway lands. He offers a litany of far-flung places, presumably that he has read about in books. These include islands such as those described in the novel Robinson Crusoe, eastern Islamic cities with "mosque and minaret," the Great Wall of China, the Nile River in Africa with the "knotty crocodile" and "red flamingo," jungles with "man-devouring tigers," and other locations. These places are all far abroad from where he is. In a spirit of adventure, he longs to leave his homeland and travel so he can see them all.
The poem "Foreign Lands" considers the theme of "abroad" from the perspective of a young child. To a child, any place beyond the familiar home and domestic garden is a far country. The narrator climbs up into a cherry tree, and to him, the garden next door, the nearby river, and the road that leads to town are all foreign lands. He then takes his perspective a step further and imagines that if he could climb a "higher tree," he might see all the way to where the river meets the sea and possibly even to where the road leads "onward into fairy land." Stevenson here writes of "abroad" not so much in a literal sense, but rather as a child views the vastness of the world.