Lots of love for these, and I like stormbron's #4 (filing that one away for later) SO how about I flesh out the top three a little more:
1: The players would be infused in toys which are appropriate for their character class. Maybe setting a goal at level 7 is artificially inflating it. Combat would be played in a much more "kid gritty" fashion (humans react a lot more to damage inflicted by the toys, almost overreacting. I.E. the "hopping up and down on one foot when stabbed by the sewing needle" thing.) Players will be "quicker" (I.e. able to outrun a human plodding down after them) and a little "tougher" (Needle and thread is enough to heal the teddy bear, and carving a fresh leg will get the toy soldier back on his feet)
2. Changing it up a little: from simply a "race" to a multiple-stage scavenger hunt. Heavy on the pulp flavor, Two-fisted heroes and heroines are theo order of the day, with more than a bit of nationalisim. There will be a definite Snidely Whiplash sort of fellow, but he won't be alone in unsportsmanlike conduct, just the biggest purveyor of it. The race will take place across land, sea, and air. (with appropriate vehicles for each.)
The other competitors will fill out the necessary "madcap hijink race" cast rolls (the flamboyant prettyboy, the professional racers, the defiant all-female team, the mad inventors, etc). Some contestants will drop out of the race before it's over (tragedy) alliances will be forged and broken. If the players wish it, that is...
On the road from point A to B, the racers will have to pick up artifacts of each civilization they pass through, through some blatant tomb robbing / pulp-era heroics. Though if they don't feel like tomb raiding, it's going to be a bit tougher, maybe some shady, criminal contacts to procure the item ahead of time? It's really just for bragging rights, and nothing makes a PC feel better than getting to the finish line with something to brag about.
3. I was actually looking at the underwater campaign as an excuse to use all the monsters in the manual that don't show up in regular play, like the merfolk and whatnot. Plus, could you imagine the party running into a party of above-ground adventurers, while underwater? It could be interesting...