Absolutely the best way to breathe life into an older computer is to replace the slow mechanical hard drive with a modern solid state drive. The computer and apps will boot 5x faster, and overall the system will be dramatically more responsive.
This MacBook Pro (Mid 2010) was pulling read/write speeds of around 60MB/sec from the stock hard drive. Not great! After the solid state drive is in (a Samsung 850 EVO), those speeds rocket up to 260MB/sec. Not to mention improved battery life. The difference is hugely noticeable – and the client was very happy.