I agree with the pp - toddlers nutritional needs seem to vary so much -- there were a few weeks about a month ago where my dd was eating pretty much nonstop -- full meals and lots of snacks in between. On a normal day, she eats a big breakfast, a good sized lunch, a few bites for dinner and has a small mid-morning snack and a good sized mid-afternoon snack. If she's looking for food at other times, I give it to her.
Healthy snacks to leave out: whole grain crackers, toast, carrots, peppers, cheese, apple chunks, cucumber pieces....will keep thinking.