Neuroscience Nursing Certification - a very popular one for neurocenters. I have a lot of respect for the nurses who have taken this test.
You can take courses in Burn care beyond the ABLS.
Cardio Vascular Nurse
Cardiac-Vascular Nursing Certification Eligibility Criteria - American Nurses Credentialing Center - ANCC
ECMO training and cert - takes alot to get and maintain unless active.
There are also courses for microsurgical care for RNs who can for replantation patients.
The above are for adults. When you get into peds and NICU you get several more speciality which might be a requirement rather than a merit badge.
Most of these RN certs require about 2000 hours of actual work experience in the specialty unit and the employer may expect you to have obtained at the very least the minimun knowledge on these exams. For the unit's accreditation, certification of all the RNs migth be highly encouraged. These certs are not for the new grad who just want more alphabet.