Dexter's Lab was indeed a success on Cartoon Network during the late 90's when it aired its  second season, and even more after the revival around 2002. Dexter's lab actually  received more fast food promotion then other Cartoon Network series at the time  

The first promotion was at Wendys in 1997. One or two of these toys can be found on amazon if you look hard.

Next promotion, while not fast food, was at Subway in 1999. These toys look pretty cool, but they're pretty hard to find, but I saw one or two on Ebay.

Dairy Queen had a promotion in 2001, I remember seeing the toys, but wasn't able to get any. They are extremely hard to find now. 

The last promotion before the series ended was at my favorite fast food place, Burger King in 2003. I remember being so excited when these toys came out. They're very easy to find. I got a Dee Dee toy from Amazon recently for a pretty good deal.