To gain extremely easy experience, get any character fast to level 1-7 (further if you wish to spend time). Short and simple, go to World Explore - Mount Fuji - Area Zero - Race it. This race is extremely easy in getting speed, staying under control, and it takes less than 1:20.
The reason this race is best is because like any map there is a new way each time to break your record, but this map is so small that your doing it in less than 2 minutes each time. The first time you beat the " gold record" try to just beat it. And each time you're trying to beat your personal best try to " just beat it". Doing so will allow you to get the " beat personal best" extra rewards which allots you to around 50k+ each time you complete race.
The great thing is, you're also getting bonuses for other various things throughout the race. Once you get better you'll boost the whole map, and you'll get tricky each time. This ups your bonus and once you beat quite a few personal records like i have, now you're looking at 100k credits (same amount of experience as well) and this will level your character extremely fast.
Fastest Route:I find when you first jump off the copter make sure you set your character up to jump below and not to the right. This will give you the fastest route i have found thus far, however keep in mind I'm still trying to "just beat" my record and not beat it by a whole lot. Stay down here and let yourself do tricks on each bump to keep your boost going, and never let yourself get carried away as you WILL fall and have to restart (otherwise you time would be affected by rewind). Just experiment and you will find what best suits you.
Beating Gold the first time requires a time of 1:32. So far I'm at 1:07 and hopefully i can keep pushing at it. It gets a lot harder and you'll find yourself racing yourself and not the other opponents in the match each time. Enjoy, and remember all easy race maps are like this, just make sure you do this one first it's by far the easiest I've found.