You can also use a circular polarizer as a 2 stop ND filter
One option is to go with a variable ND filter. These are screw on ones and turn like a CP to adjust the filter. I wish I could tell you how it works, but I have not had a chance to try it out yet, but I got one very cheap from Amazon. I assume there will be a colour cast, but it was only $21 (CDN) so I thought it was worth a try.
I do also have a Hi-Tech/Formatt 10 Stop ND filter that is great. I got the screw on one because fitting the filter holder and the rest is sometimes very slow and fiddly. I have two 5 1/2 year old daughters, I do not often have much time