I thought most of the lower-division physics courses were easy, so I'll assume you have about the same level of understanding as me.http://physicsguy.com/ftl/html/FTL_intro.html
for the basics of relativity. I didn't really understand that stuff until I read it, but it's only a primer. You need all the fancy advanced math to really grasp it, apparently.http://www.gregegan.net/DICHRONAUTS/DICHRONAUTS.html
is really interesting speculation on what a universe with two time and two space dimensions would be like. Easily accessible, really trippy stuff, I'm definitely gonna give that book a read when it gets published.