It will depend in part on what they are doing. I had limited internet when I was homeless and had to cope with doing stuff on a tablet in the evening with battery power and no internet.
Find games that can be played offline.
Use a URL to PDF converter for things you want to read.
Favor lightweight websites and lightweight apps.
Get a video downloader and download videos you like watching repeatedly, like music videos.
Clear your cache regularly and delete stuff you aren't really using.
Try several different browsers to compare performance. After finding one that works well, uninstall all others that can be uninstalled. Some mobile browsers have an optimization option that will help.