According to Wikipedia, diploma is a certificate or deed issued by an educational institution, such as college or university, that testifies that the recipient has successfully completed a particular course of study. The word diploma also refers to an academic award which is given after the completion of study in different courses such as diploma in higher education, diploma in graduation or diploma in post graduation etc.
As you have mentioned you are interested in Arts stream I would suggest some of the top diploma courses which you might find interesting.
1. Diploma in commercial arts- First of all don't confuse it with fine arts. Commercial arts is totally different from fine arts. Commercial arts actually trains and educates you on how to sell and market good and services.
2. Diploma in art teaching- This is the type of diploma course that helps you in equiping all the knowledge and training you should possess for becoming an art teacher in some collegiate institution.
3. Diploma in hair dressing and beauty culture- This diploma teaches you exactly what the name suggests, hair dressing and beauty culture. You will be taught on how can you style haircuts and all that that matters for joining a glamour industry as the beautician.
4. Diploma in stenography- Stenography is the action or process of writing in shorthand and transcribing the shorthand on a typewriter. The diploma course trains and educates you to turn into a stenographer.
5. Diploma in hotel management and catering technology- The hotel management and catering technology diploma teaches you the basics of food, accomodation and catering services.
6. Diploma in garment technology- The diploma in garment technology teaches you in the technical aspects of the production of any garment.
So, these were the top 6 diploma courses you can opt for just after high school from any reputed institution which gives you an equivalent certificate to that of a class 12th result.
Choose the option in which your interest lies.
All the best!