When you first visit Union Cave, you probably won't be able to use Surf and Strength, which you need to use to get around the entire cave. You will be able to explore the first floor and the middle portion of the first basement. Talk to everyone you see. Some people have very interesting information.
This cave is pretty simple: just head south, fighting the trainers when you must and picking up items where you can. But if you have the fortitude to handle a short detour, take the stairs on the high ledge to the left. You won't get far in the basement, but you can easily get to TM 39. It contains Swift, a strong Normal attack that always hits and can be learned by most Pokémon.
Only Gold and Crystal players can catch a Sandshrew here, but Silver players needn't despair: They can buy one at the Goldenrod Game Corner. Sandshrew is a bit better in Gold/Silver/Crystal than it was in Red/Blue/Yellow, since it learns most of its attacks earlier and has a few new ones. But it still doesn't learn any Ground attacks, which is annoying since it's a purely Ground-type Pokémon, and suffers many weaknesses to popular types because of it. But if you haven't had a chance to catch a Phanpy, this is a decent second choice.
Goldeen's a bit more interesting than it was in Red/Blue/Yellow. It now learns Horn Drill, an attack that K.O.'s any Pokémon instantly 30% of the time, at level 15 (it used to be level 45). This move, combined with a couple of X Accuracy items, could wipe out a legion of opponents if you get lucky. Teach it Surf and Waterfall early (with HM's), and Goldeen's not half bad.
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Sandshrew
Rattata
Geodude
Onix
Zubat
Wooper
Magikarp
Goldeen
Seaking
Tentacool
Tentacruel
Quagsire