I see no problem with capping it but I honestly like the idea of some real plants next to a pool. What would be the purpose of capping it IMO the tile is the real dated part.

I like the tile! I think it needs black accenting. I know it’ll get hot ,but the white is washing the tile out. I also like plants I think it just needs a landscape overhaul

I would use the planter for an all year herb garden.

Turn the planter into a fire pit perhaps. You can have seating around behind if you have room. We've converted planters like this into fire pits on older pools during remodels. Works well, and if it were constructed properly it will have a drain in it. Get you a stainless grill that would fit inside and you're in business!