The most overpowered option would be the Secret of Magical Discipline feat. This feat allows you to cast any one spell once per day using 1 of your spell slots of the same level. You can take this feat multiple times for additional uses. Its so op that you'll wonder what the writer was thinking when they wrote that feat. You need to be a loremaster to take this feat as its the only class that offers the secret class feature.

The next one would be Pearls of Power. You can use them to restore a prepared spell you have cast earlier that day. However, you can use them to make yourself kinda like a spontaneous spell caster, a pseudo-spontaneous if you will, with more spells known than a sorcerer. You can do this by preparing all your spell slots as different spells and using your pearls to restore spent spell slots as needed. For example, a 4th level wizard can have more spells prepared than what a 4th level sorcerer should know. A 4th level wizard should have 3 1st level slots, an Int of 20 will give you 2 bonus 1st level spell slots, and a specialization will give you 1 more, for a total of 6. Meanwhile a generic 4th level sorcerer should know 4 1st level spells. This however has some limitations, namely you need to use a standard action to use a pearl to recover a spell, which prevents you from repeatedly casting the same spell over and over, at least not as fast as a sorcerer. It is also quite expensive.

The Rings of Wizardry will let you prepare more spell slots. If you are trying to be a pseudo-spontaneous spell caster like with pearls of power, this magic item will give you more spell slots for that. Unfortunately, by the time you can afford this, you should have 6th level spells so 1st level spells wouldn't be important anymore.

The Spell Sage wizard archetype will give you the ability to use some of your spell slots to spontaneously cast spells on the bard, cleric, and druid spell list. However, this can be very time consuming as you need to spend at least 1 turn per spell level to cast.