Viva Piñata

Basically everything you do contributes to your experience. By getting new pinatas to visit your garden, become residents, and breed (getting master romancer helps a lot as well), you'll get a ton of experience.

Growing plants to maturity and using fertilizer for a full boost also helps, and will get you the plant achievements too.

The way I went through to do this was go into the encyclopedia (for either pinatas or plants), and took it one species at a time. Get all the ribbons for a specific species and move on to the next one.

Here's a simple checklist to use for each species:
Pinatas:
-Seen
-Visited
-Resident
-Romancer
-Master Romancer
-3 Variants

Plants:
-Grown to maturity
-Catalyze (levels 1-3)

This will be one of the last achievements you get, so go through the rest and by the time you're ready for this and the 50 hours achievements, you'll already be nearly there. Just take it one species at a time.