A car seat. One that is not expired (yes, they do expire) and that hopefully fits your car well. Less expensive ones still meet safety standards but are often difficult to install/transfer. Try one out before you buy it, if you can.
There are car seat fitting inspection stations that can help you check an existing car seat. Do a search and you may find one in your area.
One working woman I know had one that lifted out and sat right onto a skeleton of a stroller and then turned around and continued to be useful as the baby turned into a toddler. It was perfect for her commuting requirements and rural living, because that stroller was built for rugged use.