With platter/spinning drives on the way out, the days of easily getting a terabyte of storage in a laptop or even an iMac are perhaps not over, but you can certainly see the end from here. This means even if you do buy a new lighter laptop with a better processor and screen, it still might feel like a downgrade. Where can you put all your stuff? There are a few ways you can "create" more space on your machine without having to pay an arm and a leg for more storage.
First up, you can make sure that everything on your computer actually needs to be there. That app you downloaded that time because it was free? Or that app you picked up to do a specific thing for a month two years ago? Both of those can go. A utility like AppZapper (five free "zaps," USD $12.95 purchase) does a great job removing extraneous files that can be installed by applications, or you can use something like CleanMyMac (free trial up to 500mb of files, then $39.95) to help sort out what you do and don't need anymore.
AppZapper can help you uninstall apps completely.
You can move your iTunes Library to an external drive, or your iPhoto library. Keep in mind that if you do this on a laptop, that data isn't available when the drive isn't plugged in. So if you take your machine with you and want to listen to music in iTunes often, then maybe this isn't a good solution for you.
Another thing to check is whether or not you've cleaned out your Downloads folder. Since it doesn't get emptied automatically, you can accumulate quite an assortment of files that just sit there taking up space. Also double check those folders on your desktop; if you periodically do a sweep and dump everything in a folder, that can be eating up space as well.
I might have less items in my download folder if I stopped buying book bundles.
Next you can stop syncing all your stuff to all your stuff. Dropbox and Google drive offer selective sync, and other services offer a similar feature where not everything you have stored "in the cloud" has to be on all your devices. While that seems like a huge advantage of cloud storage, it can hinder you if you know you don't need that folder on this particular machine.
To look for the less obvious offenders, try a utility like OmniDiskSweeper, a free utility that will show you your hard drive in terms of the space each directory takes up. You can track down forgotten items that take up loads of space (mail attachments, anyone?), and maybe even find some files you've lost.
OmniDiskSweeper showing 400MB of Messages attachments.