How can I enter hours for one tasks and days for others in the same project?

You could try having one field for days and one for hours and using a formula field to consolidate it for you

e.g.
 - "Hours" number field
 - "Days" number field
 - "Consolidated Hours" formula field that either outputs the number from the "Hours" field if it's populated, or if the "Days" field is populated it outputs the value from "Days" * 24 to give the number of days as hours, allowing you to use this field in calculations