Essentially, the main contrast between Ulysses' past and present is that, in his past, the king was a mighty warrior and adventurer, while, in his present, he is an aged statesman bored with his idle existence and unhappy with the companionship of his family and subjects. One of the major points of Tennyson's "Ulysses" is to describe this contrast, and it quickly becomes very clear that Ulysses glorifies his legendary past while regarding his present situation with distaste. For instance, take a look at how Ulysses describes his past exploits:
Much have I seen and known; cities of men
And manners, climates, councils, governments,
Myself not least, but honour'd of them all;
And drunk delight of battle with my peers,
Far on the ringing plains of windy Troy. (13-17)
From this description, it's obvious that Ulysses' past was filled with adventure, epic warfare, and great deeds worthy of legendary heroes. To contrast this thrilling description, take a look at how Ulysses describes his present:
It little profits that an idle king,
By this still hearth, among these barren crags,
Match'd with an aged wife, I mete and dole
Unequal laws unto a savage race,
That hoard, and sleep, and feed, and know not me. (1-5)
It's clear that Ulysses regards his present life as boring, meaningless, and lacking in passion, and this dull assessment is emphasized by the king's sentimental assessment of his heroic past.
All in all, the main contrast here is that, in the past, Ulysses was a mighty warrior capable of great deeds, but he is now reduced to an old man who feels confined by his duties and his age. Thus, this poem is really about the aging process and how an elderly individual deals with the inability to live the life he enjoyed in his youth. By presenting this process in the legendary context of Ulysses and The Odyssey, Tennyson dramatically highlights its inherent difficulty.