The alternative to multiclassing is a gameplay style called Gestalt. Hallelujah, the choirs are singing, I can kick tush! Originally from the WotC supplement Unearthed Arcana, gestalt is a format that HAS to be applied to the whole game to work. In a gestalt game, the characters don't just settle for one class. There's no more choosing between two classes or levelling a class or a prestige class. Gestalt lets you do both! At level 1, pick two classes you want, any two classes. From there on out, you take the best of the two for saves and stat increases (ex: if one class uses a d6 for hit dice and the other uses a d8, you'll use the d8), all of the class skills available to each, and all of the unique abilities for each (Rogue/Wizard gets sneak attack AND spellcasting.) The same applies to prestige classes, but you can only take one prestige class at a time. You have to level either C/C or C/PC, no PC/PC. Overall, your character ends up more powerful, more capable, and less constricting than a single-class character. This should go without saying, but you can't just double up on levels from one class for a level. For example, you would have to take a level of Ranger and a level of Fighter, you can't take two levels of Ranger or two levels of Fighter. That's just silly.
The downside of this is that if one character takes levels as a gestalt, it's much more difficult to detemine the difficulty of enemies. Something that would have been a threat to a single rogue can be wrecked by a wizard/rogue, to use that example again. It's up to the DM to balance the encounters once your team goes gestalt, either by adding more combatants, upping the difficulty level of individual encounters, or gestalting the enemies as well. The other downside? Once you go gestalt, you can't go back. Just try playing a normal character again. It's