Hmm... so she won't let you use it as a planter, and she won't let you sink it into the ground as a pond... but she's happy to see it sit there covered... which must mean that in some part of her mind she's thinking it can come back to life as a hot tub...

Hmm...

Well... all I can do is tell what I would do.

I'd plant some shrubs around it, but still leave the steps into it open, and I'd fill it up with water, and I'd go a local pond and scoop out some buckets of mud in order to get a good sampling of all the little microranism that live in ponds, and seed them in there, and I'd seed it with fresh water shrimp, and I'd also collect a whole bunch of pond weeds, and sow those in there, and then I'd got to a fish hatchery and buy some rainbow trout fry, and plant them there... ranbow trout can live and grow in still-water about as deep and wide as hot tubs get.

I'd let them feed on the fresh water shrimp, or toss in fish pellets (guinipig pellets, or dinner leftovers will do too) if it looks like they're not getting enough from the fresh water shrimp, or if they've eaten the shrimp to extinction, and when they get big enough... I'd start fishing for them... you know... flyfishing from behind the bushes that were planted around the tub... or maybe play games, like... walk up the stairs to the edge of the tub and see if I can catch them with my hands.

That's basically what you're thinking of... turning it into a fish pond... only without half burying it... just ringing it with shrubery, and she can't say that you're turning it into anything that could not, in theory, be turned back into a hot tub, but by ringing it with shrubery, you've fixed the eyesore aspect of it.

You might get lucky and have some mallards adopt it, and you can even have a fishing party with your pals, only with easy access to the fridge.

That's what I'd do.