You should study "The Algorithm Design Manual" by Steven S. Skiena. It's really good and it will teach you what CS students learn in the Algorithms and Datastructures class, including evaluating if the runtime of a program is going to be quadratic, linear, n log n etc. in worst case, best case, average case.
You should read a bit about patterns, at least so far that you know what the most common names refer to. One thing is it's good for your design and programming, the other is it's a tool of communication. You may also like a book about antipatterns.
Other than that it depends on what your current programming experience is. E.g. some people develop great non-object-oriented code for years and never really get the concept of OO, if you are one of them try to find a website that teaches OO concepts in terms of one of the programming languages you are fluent in (unless you are and almost certainly will be in an area where OO is completely irrelevant ... stuff close to hardware or something). Or if you e.g. ever only used the little bit of SQL that is enough for many programs (select etc.) you might want to find an SQL tutorial with exercises, skip the parts you already know and get into let's say joins, if you never used those before. So in general: Go a little bit further than what you have learned so far by always studying exactly what was needed to get the job done.
If, for your previous work, you never used any kind of modeling, e.g. UML, you may want to look into that a little bit - you could miss out on jobs where this is used extensively in the requirements or design documents, e.g. on demand from the customer.
Depending on what you have been doing professionally and where you want to go you may want to take a little bit of business/economics classes, like accounting. I hated it like hell when I was studying CS and it was mandatory, but it turned out to be one of the very few things from college that are somewhat relevant for my job.
If you don't know what people mean by "agile development" or have only a very vague idea, you may want to read up to find out, also on some of the various methods, so that you are not stumped if the topic comes up in a job discussion.
I don't recommend going out and studying computer science. It will cost you a hell of a lot of money (at least if you're in the US) and not teach you a whole lot. So unless you have money lying around and also no problem that you will earn very little for 4 years, in which case you could do it for the fun and experience, it's not really worth it for someone who can already program well and gets properly-paid jobs without a degree.
You should read a bit about patterns, at least so far that you know what the most common names refer to. One thing is it's good for your design and programming, the other is it's a tool of communication. You may also like a book about antipatterns.
Other than that it depends on what your current programming experience is. E.g. some people develop great non-object-oriented code for years and never really get the concept of OO, if you are one of them try to find a website that teaches OO concepts in terms of one of the programming languages you are fluent in (unless you are and almost certainly will be in an area where OO is completely irrelevant ... stuff close to hardware or something). Or if you e.g. ever only used the little bit of SQL that is enough for many programs (select etc.) you might want to find an SQL tutorial with exercises, skip the parts you already know and get into let's say joins, if you never used those before. So in general: Go a little bit further than what you have learned so far by always studying exactly what was needed to get the job done.
If, for your previous work, you never used any kind of modeling, e.g. UML, you may want to look into that a little bit - you could miss out on jobs where this is used extensively in the requirements or design documents, e.g. on demand from the customer.
Depending on what you have been doing professionally and where you want to go you may want to take a little bit of business/economics classes, like accounting. I hated it like hell when I was studying CS and it was mandatory, but it turned out to be one of the very few things from college that are somewhat relevant for my job.
If you don't know what people mean by "agile development" or have only a very vague idea, you may want to read up to find out, also on some of the various methods, so that you are not stumped if the topic comes up in a job discussion.
I don't recommend going out and studying computer science. It will cost you a hell of a lot of money (at least if you're in the US) and not teach you a whole lot. So unless you have money lying around and also no problem that you will earn very little for 4 years, in which case you could do it for the fun and experience, it's not really worth it for someone who can already program well and gets properly-paid jobs without a degree.