Are you considering applying to do a PhD or a Masters? If so then here is a really great place to ask for advice. There are quite a few people who are doing or have done a PhD on this forum.
From what you've said so far (namely the point about challenging yourself with further study and expanding your career options), it might be a good idea to map out what possible careers would be of interest, and see if Masters are available in or relating to those areas. I know that there are Masters for things like Business Management, and sometimes you only need to have a relevant qualification (i.e., not necessarily a degree) plus relevant experience to meet the criteria. My friend and I were looking at some of these just the other day. There seems to be a variety of options - including online and part-time study.
Hope this helps as a start!