So you're not too concerned with a complete inventory, then? Getting the summons and Pink Tails is the easy part, really. There are plenty of enemies that have items just as rare, and sometimes are much harder to fight, either because they can't be conveniently summoned with a Siren, or because they're just really powerful. Here's a list of all the rare drop items you may not have:

Cursed Ring - Souls and Spirits drop it as their rarest drop. Easiest place to get this is Mt. Ordeals.
Lilith Rod - Liliths drop it as their rarest drop. Easiest place to get this is also Mt. Ordeals.
Giant's Gloves - Steel Golems drop this as their semi-common drop. Easiest place to get this is Cave Eblan.
Rune Staff - Marionetteers, Sorcerers, Summoners, and Mist Summoners drop this as their semi-rare drop. Easiest place to get this would be either the Tower of Babil or Tower of Zot, but at the end of the game the only place you can get them is the Feymarch or the Lunar Ruins.
Summon Book - Marionetteers, Sorcerers, Summoners, and Mist Summoners drop this as their rarest drop. Easiest way to get one, however, is checking the bookshelf in the Sylph's house after obtaining Bahamut (sources vary on exactly what triggers the book to appear on the shelf, but this definitely will guarantee you've done everything to trigger it).
Stoneblade - Black Lizards drop this as their rarest drop. Easiest place to get this is roaming around the map in the Babil region of the Underworld.
Rune Axe - Armor Constructs and Armored Fiends drop this as their rarest drop, and the Iron Giant drops it as it's semi-rare drop. Easiest place to get this is the Lunar Subterrane from Armored Fiends (Iron Giants are just too rare to effectively farm)
Artemis Arrows and Artemis Bow - Common drop and rarest drop from Moonmaidens. Arrows aren't a problem, but the bow is what I feel to be the hardest item in the game to obtain, due to Moonmaidens not being Siren-able.
Wyvern Lance - Rarest drop from Blue Dragons, but semi-common drop from Red Dragons and Crystal Dragons. Easiest way to get this is farming Red Dragons in the final floors of the Lunar Subterrane (the crystal tiles).
Dragon Whisker - Semi-rare drop from Blue Dragons, but semi-common drop from Fiend Dragons. Fiend Dragons in the Lunar Ruins are the best bet for getting this.
Crystal Ring - Red Dragons and Crystal Dragons drop this as their rarest drop. Like the Wyvern Lance, easiest place to get this is the final floors of the Lunar Subterrane.
Power Sash - Behemoths drop this as the semi-rare drop and King Behemoths drop this as their semi-common drop. Behemoths on the final floors of the Lunar Subterrane are easier to farm than Kings due to Kings being very rare.
Glass Mask - Deathmasks and Proto Deathmasks drop this as their rarest drop. Deathmasks are actually easier to farm due to be Siren-able and being easier to kill due to wasting an extra turn casting Reflect on the party.

That should be every other rare drop in the game worth obtaining. If this is too much work for tyou, don't even attempt to go for a 100% inventory or Tail-hunting in general in The After Years. It's 10x worse in that game due to there being no Sirens, waaay more rare drops, and much stricter requirements for Tail-dropping enemies to appear.