Pu erh is pretty common these days… But what is it that you want replacing? Energy? sugary is your alt so maybe teh tarik (if mamak drinks got). Anything with ginseng if it’s not too heaty for you. Mouthfeel? Then just switch to decaf (if kopitiam got). Just lower caffeine? cold green tea or cham.

If you want taste to your plain water, ask for air limau but kosong (no sugar). If you can get the shops that would layan you adding a bit of salt you’ve basically halfway to an isotonic solution lol