Okay, so I have played around with various designs and thought about the problem a bit.
First off, a disclaimer. These are NOT dolphins. Rather, they are dolphin-like creatures that could evolve on another planet. They couldn't originate from dolphins, because dolphins don't have the necessary requisite parts for these structures to evolve from. So what is shown below is merely an imagining of possible manipulator appendages that could exist on a dolphin like species.
What I reasoned was that having the forelimbs (or flippers) evolve to act as manipulators was a bit unlikely, as they are typically too far back and help in steering and movement, having to use them for object manipulation would be awkward and inhibit movement - a bit like trying to use your feet to carry something. So I decided that a mandible or mouth configuration would be most probable. Even pre-sentience, dexterous mouth parts would assist in catching prey and various other tasks.
There is some precedent for such mandibular manipulators. Take for instance, the pedipalps found in spiders and some arthopods, acting like a tiny set of legs or sometimes claw like appendages that help capture and guide food into the mouth, sometimes even tearing pieces off or holding things. There is also the mouth flaps of the manta ray and the tentacles of a cuttlefish. I took inspiration from some of these things, which I will show below.
The mouth flaps of the manta ray, called cephalic lobes, are wing like, and streamlined, and help guide food into its mouth.
The cuttlefish shown here, like many cephalapods, has very dexterous tentacles which can be used in any direction, lengthen, contract and curl to grasp objects in it's environment. The cuttlefish moves both forwards and backwards. When moving forwards, the tentacles are often pressed into a cone-like "torpedo" shape for low drag.
This strange creature, called Anomalocaris, was one of the apex predators of the Cambrian period, long before dinosaurs. It had curled flexible appendages with needle like bristles facing inward to catch prey. I liked the configuration of frontal appendages because it seems like it would be useful to manipulate things in front and below the head where the eyes could see everything.
So finally, I would like to present my take on a tool using dolphin like species.
Some of the early versions that I tried to make that used a split jaw like a gulper eel or other bony jaw structure turned out quite... disturbing. Like the unholy lovechild of the predator and a xenomorph with the body of a barracuda. Seeing as you were looking for dolphinesque creatures, and not horrors of the deep, I switched my efforts to other designs.
The best of what I came up with were based in large part on the three animals shown above.
(Note: in my drawn designs, all except for one, in which I wasn't thinking about it, I tried to preserve the rounded head of the dolphin, as underwater echolocation seems to require a resonating chamber near the front of the head, which seems like an extremely useful feature.)
Also, I decided that having especially long reach was not as important, as wide swinging motions are not as useful or efficient underwater. Trying to use gravity and momentum for something like swinging a hammer just isn't very practical underwater. So manipulators don't have to be long, but having enough reach to carry a long item to either side of the head or below the body is a good idea. When underwater, the types of motions that would be most useful would be hooking raking or scraping motions, poking and jabbing motions, being able to grasp and pull towards the body or mouth as well as tear and pull an object apart, or to simply hold onto an object and hold it close along the body for carrying something without creating too much drag.
You can see the manta ray influence on the top one. It's one of the more aesthetically pleasing designs, I think. I first thought of having the manipulators be folding like a curled flipper or mitten. It is streamlined in the direction of the water flow though. As nice as it looks, The lobes leave a lot to be desired when it comes to fine motor manipulation.
Below it, you can see a design more inspired by anomalocaris, with the mandible manipulators in a closed, low-drag swimming configuration. It has a bit of a curl or hook that tucks under the jaw. Instead of soft jelly like muscles, you'd want something a bit firmer than the average cephalapod tentacles which would help brace itself against high speed water flowing over them while retaining their shape.
So this sketch shows a possible nautilus inspired design. It came out looking more "fishy" than I meant for it to. A nautilus is pictured below for reference; it is a bit like a cuttlefish but with a shell and a trapdoor-like "hat". My idea here was that soft tentacles, (assuming tentacles were desired,) would be a source of drag, especially for something as streamlined as a dolphin body. So in this design, the tentacles would be retractable and stored behind a closed "nosecone" of sorts. I'm not sure how much I like this design, but I thought I'd put it up here for variety.
A nautilus, for reference.
** And my final design **
After some consideration, I returned to the second design after making some slight modifications. The manipulators are shown here in both retracted and extended positions. Having the eyes positioned so that they can both see somewhat forward for seeing what it's doing in front and sideways for keeping an eye out while not being obstructed by the manipulators was a bit of a tricky problem, but Having the eyes placed close to the outside slanted forwards a bit, with manipulators protruding under the eyes seemed an appropriate placement without resorting to eye stalks or anything too exotic. Whether you imagine these manipulators as more soft like tentacles or more rigid like the anomalocaris, I imagined that they could lie flat against a protruding beak like structure that would allow the manipulators to rest and fold in a streamlined position, without needing to use muscle strength to fight against the current. If the manipulators are soft, the hard beak makes for a good tool to do things the soft parts can't and vice-versa. It could also act as a "third hand" which could hold a piece of work while the more dexterous manipulators work on it. And the dolphin's characteristic head bulge has been preserved, giving it room for a resonating chamber allowing it to use echolocation, which would be an extremely useful ability.
Edit: Ok, so second final design, after looking at the comment by FrankRebin, I decided to rework the nautilus-dolphin design to be a bit more streamlined and blend with the body better. I figured I'd just tack it on the end here for good measure!
Just a rework of the final design without overhang on the tentacles and a bit more streamlined.
So those are the best of what I came up with. There are a few things I didn't put up here, namely the designs that used more bony protrusions and teeth as manipulators, mainly because they were decidedly un-dolphinlike, and kind of scary looking. They would make much better sharks or deep sea predators than dolphins. I hope you find this interesting and gives you some helpful ideas. I would appreciate any feedback or thoughts!