My wife and I want to fill our bathtub, and do some IG photos and just have something to play in. Looking suggestions of cheap substances that will last several days without going bad. I want to be able to play in it on/off for several days. Thinking ahead to ease of cleanup also.

So far we've come up with maple syrup, jello pudding, or oatmeal. Any advice?

Water

If it's real food based, it will go bad. Oatmeal or pudding will probably start to turn after a day or so. Maple syrup (which is usually made from high-fructose corn syrup) would be a safer bet, as it has less 'real food' in it.

The best option might be to buy some gunge powder and make a mess out of artificial ingredients. Even some of these come from natural sources, while others are polymers. A polymer slime would last the longest, as there is nothing to go bad.

Also, be prepared for icy cold, as any substance allowed to cool down to room temperature will feel quite cold. If its thicker, the cold sensation will pass, as the substance itself will insulate you somewhat. Lastly, make sure it will go down the drain once you're finished with it. Long-chain polymers (artificial slime) may be broken down by adding salt.

Pancake syrup (as opposed to 100% maple syrup) is probably your best bet here. It keeps for a while at room temperature, is a nice consistency, and will go down the drain. Slowly at first, but with hot water, it soon dissolves and clears. It has a really good feel for coating and filling clothes, plastering down hair, I'm not so much into naked WAM, but I've heard good things for that too.

Two cautions: it can sting eyes, though not as bad as honey. And be sure to clean thoroughly before leaving the tub area. It feels really nice in quantity, but a stray drop becomes very sticky, which then becomes dirty, attracts ants and can be difficult (but not impossible) to get out of carpeting.

Be sure to post links to the IG photos for all the lonely and jealous folks out here.

slime baff works for a long long, up to 30 days, get a small pool

I'd suggest methyceluose based gungeslime, you can mix it as thick and sloppy or thin and gloopy as you like and it'll go down the plug hole afterwards. Also none toxic and it holds its colour.
Trust me, I've tried porridge and it can be a nightmare to get rid of afterwards

Thanks very much!!

Back in the early days of WTFdownloads I filmed two scenes with smooth oarmeal (Ready-Brek style) on sequential days, but by the second day it was a bit smelly and BUBBLING, and stickier.

That didn't actually deter Heather from going in head first and then bathing in it too. She was nuts and would get messy in anything at every opportunity though.

Compare the consistency of the porridge on day one (Bubbles' scene) vs on day two (Heather's scene). That's only really clear in the video, but here are two pics of Heather on day two.

https://wtfdownloads.umd.net/download_info/bubbles_and_heather__porridge
https://wtfdownloads.umd.net/download_info/heathers_porridge_bath

We also use Home Made Cake Batter: -
Flour
Eggs
Water
Cocoa Powder

Mix it up as thick as you like and easy to get rid of

Dang! How did I miss Heather! She was freaking hot.

i not had any food stuffs left around for long lasting wam, though syrups and chocolates seem to last good,

as for the 'keeping it warm' issue someone mentioned:

if you have a 'spare' bath for gunge you could use a soil heater (gardening) for keeping or warming up the bath of fun, another option with cold gunge/mess over a few days is to build in a hot water pipe running around the exterior of the bath like a coil, have that plumbed into your central heating or hot water supply (pretty easy install)

or a very simple way to heat up gunge is to fill 5 liter containers with boiled water then place them in the bath with the gunge, the heat transfer soon warms up the gunge without diluting it

I mix the cake batter up in 12 litre buckets and use hot/warm water then you finish up with industrial amounts of lumpy sludge at pretty much the correct temperature