I have been getting a lot of utility out of the changeling racial sub levels for wizard along with some of the gnome illusionist sub levels (Just don't trade off your familiar) - if you can stack on additional racial subs, that could get even more interesting.
Cleric sub levels and ACFs tend to all trade off the same thing, since the cleric only has one class feature, so you'll probably find a lot more of interest if you look through druid levels, as you're more likely to be able to stack them. Half Orc druid 1 (RoD) is free bonuses, and the later half orc levels are very strong as well. Halfling druid 1 (RotW) loses spontaneous nature's ally for the first few levels, but gives you basically free awesome bonuses. Even with just the half-orc levels and the halfling levels except 5, if you take able learner, you get climb, jump, hide, move silently, and intimidate as class skills, get 6 skills/level, and get assorted bonuses to summons and animal companion at basically no loss except for taking two good feats and being a changeling (never bad). Check the druid handbook for more levels to stack on as well. (Shifters also get racial subs for druid, not sure who else does among the humanoids, though.)
