Consider taking a prestige class that continues your wizard spell progression instead of a completely different class.
You get level 9 spells at level 17, not 15.
Which sources did you check for Prestige Classes? What sources are allowed?
Keep in mind that, by multiclassing out of Wizard, you lose more than access to higher level spells -- you also lose spell slots for all levels, and your Caster Level goes down, lowering the effectiveness of your spells and your chances of succeeding on concentration and spell resistance checks.
What does "Pathfinder Approved" mean? That you can use prestige classes in sources published by Paizo? Have you looked through all of them?
You're a Wizard, you have a bazillion skill points - that's your flavor right there. With skills and maybe a feat or two, you can add a lot of flavor to a Wizard without destroying your power progression.
A level of Paladin or Gunslinger will add very little to your character. As you said, armor isn't an option and as a Wizard (primary) you're not hitting things with guns or swords.
If you want to hit things with guns or swords, take a Weapon Proficiency Feat. One level of Paladin or Gunslinger won't help you hit things much better anyways, but you can play Wizard with a Gun quite well as a straight Wizard.
I've got a 13th level Wizard going right now. He can sneak, survive in the woods, appraise goods better than most Rogues, is a master of alchemy and a decent painter. He's also an able diplomat and a passable merchant, can climb more than a knotted rope, is a capable rider and can even drive a wagon if the need arises. Wizards get a ton of skill points.
I recommend a level of ranger. (Of course, you might want to keep in mind that I'm a huge fan. Very biased.) You could make your character very different from the stereotypical academician with just one level -- especially with the trapper archetype from UM.
It would add 34 to your skills. (I don't mean 34 total; I mean 34 more than from another level of wizard.) Four more skill ranks than a wizard, and a boatload of new class skills: Climb (Str), Handle Animal (Cha), Heal (Wis), Intimidate (Cha), Perception (Wis), Ride (Dex), Stealth (Dex), Survival (Wis), and Swim (Str). Oh, and Disable Device (Dex) from trapper.
It would add several new class features, including a bump to tracking and a versatile bonus against a favored enemy. (I recommend humans.) Even if you never use a weapon against your favored enemy, you also get +2 to bluff them (or pass secret messages), identify their accents with knowledge (geography), notice them when they're sneaking, sense their motives (or detect secret messages), and track them. Oh, and then there's the ability to disable magical traps (from trapper).
All of a sudden, you'd be a physical, outdoorsy, sneaky, tracking, lock-picking, trap-disabling wizard. All for the low low cost of reducing your spell progression to a sorcerer's. I think that's pretty cool -- but of course, like I said, I'm biased. ;)
Also, none of the prestige classes strike me as being any good and adding much flavour to my character.
Have you looked in Paths of Prestige? There are some very flavourful PrC's in there.