18 month old has fever (~99 to 100) for past 2 days, appetite ok. on tylenol, (acetaminophen) what next?

Keep hydrated: Lots of liquids and keep them comfortable. Liquids are more important than getting in solid food right now. Count number of wet diapers and if decreased this could be a sign of dehydration. Use tylenol (acetaminophen) for discomfort but not to treat the fever, fever isn't dangerous. If your child has a fever (greater than 100.4) for more than 3 days make an appt with your pediatrician for an evaluation.

Call you doctor: It's never a good idea to post questions about an acute illness your child is dealing with online because there can be a delay in getting an answer. In general, you're better off discussing these illnesses with your physician's office because sometimes there are other things going on that merit immediate attention.