replace the small hard drive with one twice the size @ 7200rpm.
You will definitely notice the difference.
A reasonably sized SSD would be expensive and probably not cost effective.
I just did one for a customer, 4GB RAM and a 500GB hard drive for under $120
My daughter has an old MacBook 2.1. It has only 1GB of memory and an 120GB mechanical hard drive. I was thinking about increasing the ram to the max 3GB and putting in a new SSD drive. I think it's SATA 1 so I'm not even sure I can use an SSD Drive. Is it worth it or will I just be wasting my money on this old mac? Processor is a Core2 Duo 2.16Ghz. It works fine just runs slow. The OS is Snow Leopard.