A.K.
I say while the baby is still in the infant carrier carseat, go with a "Snap-n-Go". Kolcraft, Graco and Baby Trend all make them. Test your carseat out with them first before buying. I took my carseat to BabiesRUs the other day and tested it. They didn't have the Kolcraft, just the other two. Oh, and MacLaren has one too. Sometimes they're called "universal". They are made to fit with many different infant carriers. You really can't get more lightweight than this kind of stroller- it's pretty much just a frame to hold the carrier (with a basket underneath). You can find them cheaper on craigslist.
After that (when the baby grows out of the carseat), I'm not sure. I love our Jeep 3-wheeler with the plug and speakers for music and it was pretty cheap- only like $70 when we bought it 4 years ago. It is big and bulky, though.