If you aren't going to be using an amp you need to look for speakers that have a high sensitivity. Speakers that have a high sensitivity means that they can get louder with what power you give to them. It's measured using 1 watt of power something like 90db up would work great. I'm assuming you don't want to fuss with mounting tweeters? If so then get a 2-way, or coaxial, speakers which means that the mid-woofer and tweet's are mounted on the same plane and assembly. Your best bet is to go to a shop is listen to a few but make sure that the head unit's powering them are in fact powering them and that the EQ's are zeroed out so that they all, supposedly, sound the same.
I'm a big fan of Polk Audio for great sound with low budget. http://www.crutchfield.com/p_107DXI6...50s.html?tp=95
You should have 6x9's stock so you'll either need to make or buy an adapter ring to fit 6.5's. If you buy them from crutchfield they will send you one for free. They will also give you speaker terminal adapters so that you don't have to cut and splice your factory wiring just plug in and go.