How to add SSD in a very old laptop (Acer Aspire 5920) from 2007

My younger siblings were using this unbearably slow laptop when I decided to help them make it faster. I thought why not make a YT video? And I did. The SATA HDD in this laptop was giving speeds of about 40MBPS only (SSD offers around 10 times more of that). Therefore I decided to put an SSD in it and unsurprisingly it becomes useable again. This laptop is from 2007, has intel core2duo 1.66ghz processor and 4gb RAM. I installed windows 10 pro which is obviously heavier than windows 7 and it still runs so smooth. Now I don&know what to do with the HDD I got?. Any ideas?