You can remove the multiclass XP penalty by playing a race with one of those as your favored class (like Elf), or as Human, where your class with the most levels just doesn't count no matter what it is. Human is pretty much always a good choice anyway.
And that's if your DM actually uses those rules. In my experience, quite a lot don't enforce those XP penalty rules.
Class Choice - Look at Beguiler
Sorcerer would work as a first class choice. Beguiler (Players Handbook II) would work even better, as it's also a spontaneous caster, only one whose key stat is Intelligence (same as the Wizard). This plays off a stat you already need and fits the idea of a "learned" character who also has innate ability.
Add Ultimate Magus as a Prestige Class
If you go Beguiler/Wizard or Sorceror/Wizard, you can then go into the Ultimate Magus (Complete Mage) prestige class, and boost spellcasting from both at the same time. One would go up faster than the other, but the class also lets you use spells from one to power metamagic from the other, and it can do some pretty neat things. See also What exactly are Prestige classes, and what purpose do they serve?
Instead of just taking the spontaneous spells as a one off thing, this lets you develop both of them as the character learns to blend his two types of magic together. It's a pretty neat (and powerful) concept.
This answer has some useful information on how to get the most out of Ultimate Magus.
Wizards can get the ability to cast any Divination spell spontaneously by trading their 5th level bonus feat for this ACF. It does what it says - you can trade any prepared spell you have to spontaneously cast any Divination spell you can cast of equal or lower level.
Other Option - Reserve Feats
If you don't want to multiclass but still want some spontaneous abilities, reserve feats are another option. These let you use some power so long as you have a spell "in reserve" that you don't cast. Examples are things like being able to do small flame strikes, shoot lightning, summon elementals, create force darts, or do some minor magic disruption. You can get that as early as level 3, and you can use them infinitely so long as you don't cast the reserved spell.
It's not as versatile as true spontaneous casting, but it also doesn't require multiclassing at all. So I felt it was wroth mentioning.