One upgrade is to increase spindle speed. Typically you'll have a 5400 rpm drive from Apple unless a faster drive was ordered (look up your drive's number on the Hardware Report on About this Mac). A 7200 rpm will give you better access times but will come at greater power consumption (shorter battery life). You'll be trading cost and capacity for speed with solid state drives (SSD) as they are typically smaller capacity but have no moving parts to wait for and may have lower power consumption. Hybrids (SSHD) combine both, and of this writing, go up to 1TB. They also will come with 5400 rpm speed for lower power consumption than 7200 rpm and will use the SSD part for frequently used data to increase speed. Just make sure the SSD or SSHD is compatible with your Mac and that any drive selected is no taller than 9.5mm or your bottom cover won't close.