My first question would be: why do you use "off the top of my head" so much?
For example, if you're typing at your computer and a coworker wanders in and says, "My screen went blank, what's wrong with my computer?", you might reply, "Off the top of my head, it could be...". But you could just as well begin straight with, "Well, it could be ...", and if you're not worried about a few extra words, you could end with, "... I can't really tell without investigating it and I'm busy right now."
So I think the meta-question here is what is the context and what is your goal; are you over-using a specific phrase or are you over/mis-using a concept? Are you trying to minimize the other person's question, implying that it's easy to come up with counter-examples? Are you wanting to qualify your own statements to protect you from being wrong? ["I told you, it was off the top of my head."] Are you trying to minimize conversation?
Depending on context, you can easily get the concept across without breaking out any little phrase or finding a host of synonymous phrases. ["I can think of two or three examples, and I'm sure that if we had the time, we could come up with more." Or, "I can think of a couple of trivial examples that illustrate the problem, though I'm sure actual problems would be more subtle." Or, "A couple of things to consider would be..."]
So, if you're wanting to simply qualify your answer as absolutely your first ideas with no real analysis, you can say things like, "A couple of possibilities spring to mind", or simply be explicit, "My first thoughts are..." or "I haven't had time to think about it, but my immediate thoughts are...", or "My first impression is...". But it really, really depends on your context and it's possible that particular phrases aren't needed at all.