I have create a new Custom Field , and the datatype is datetime. I wanted to use this field to let users define the exact date and time for an appointment. When I drag the field on a form, I can select the Field type, but I only see a date option, and not a datetime. Is there an easy way to let users select the date and the time and store it in my defined datetime field? Or should I split this up , create a date field and a time field? And use an option list for the time input? Proposals?

The easiest way to do this will be splitting up as you noted, using Select as field type and listing out the time slots as the field value options. I would create a custom field String type "Appointment Time" and provide those options to your users. This will also give you more control over when appointments are scheduled.If you need this information relayed to Sales you can use tokens such as {{lead.Appointment Date}} and {{lead.Appointment Time}}.