You can do everything but ab-initio PPL training and Night Qualification.
Tailwheel
Constant Speed
Retract
Pressurised
Turbo
EFIS
SLPC
You can do an IRI Course and teach for the IR and the IMCr. You can do a type rating course and teach for SPA Type & Class Ratings.
You can convert the holders of NPPL from ML to SSEA and vice versa.
You can carry out the 2 year Instructional flights. You can do type conversions, training for pilots who have a lapsed licence in preparation for an LPC.
You can do a CRI (ME) and teach for the Multi Engine rating.
If you have more than 800hrs P1 you could apply to become an LAA Coach and with it become a Revalidation Examiner and sign certificates of revalidation by experience.
With enough teaching time and a need you could become a Flight Examiner and carry out LPC/LST test for a variety of purposes and become an IR Examiner able to do IR revalidation's. If you do the GRI certificate you could become a GRE and conduct the PPL & IMCr ground exams.
There is more, but I think you should be able to get the drift. Whilst I hold an unrestricted FI as I work in an AOC and TRTO environment I only ever use the privileges of my CRI (SE/ME) and Examiner ratings.
If you are looking to get involved with GA to put something in rather than wanting to work in the ab-initio field the CRI is a superb rating. Just bear in mind that there is less involved in gaining the rating and thus the onus is on you to ensure that your skills match the privileges that you will gain. Small steps rather than giant leaps.