I am fond of doing fastfall Bairs to secure the kill, usually by hopping over my opponent's head and getting them from behind - the low landing lag and the push it inflicts on shield makes it safer than other aerials on block, and since Ike's Bair is incredibly fast it frequently takes opponents by surprise.
Don't be afraid to go after opponents offstage for a Fair or Bair kill, either - getting hit by one of those out there is lethal.
Utilt has high kill potential if you can catch a rolling opponent with it, but it leaves you very exposed if it doesn't work. I don't frequently find many opportunities to punish with it due to how slow it is, but it's still an important option to remember.
Uair's long active time is great for finishing off opponents above you. Try to bait airdodges and punish them with it, and even if they slip past you, you might still be able to hit them with it by fastfalling it midswing.
Usmash is great for punishing landings after knocking them into the air - more often than not an opponent will try to airdodge it, especially if you've positioned yourself well, so charging it a moment longer to punish them almost guarantees a kill, and the lowered hitbox Ike takes to use the move also frequently messes with attempts to stop your attack.
Also? Edgeguarding with Eruption is just the best thing ever.