I had lots of those cubes left over from dismantling pallets all the time, and they were just piling up. I couldn't think of that many things to do with them besides using them for places to let things rest on or against as paint dried, or polyurethane cured, until I saw my dogs playing with one and tossing it around the yard.
I looked at the cubes, and realized they were almost "square" (not perfect - but good enough), so I sanded them down and chopped them with a miter saw until they were square-ish. The beauty of this particular idea is that the cubes don't have to be perfect - they'll be beaten around in the yard if you do a good enough job. They could be turned into yard dice!
After squaring them up, I used a router to round over the edges on all sides of each cube. I realized I had two distinct sizes of cubes - one about 4" across and one style about 3" across. This will matter later if you decide to do this.
I patched/glued any significant defects such as large cracks, etc., and once that dried, I did a simplified way to find the centers and then align the holes. I drew an X shape from corner to corner. The center would be either the #1 or the center of a #5. I then used a small ruler, and let the width of the ruler be my guide point from the center of the cube and after drawing the X, I'd then draw an "H" shape. That pretty much gave me all the important points - other than dividing each leg of the X into quarter pieces for the #2 and #3 sides. I marked up all the cubes like this, so they were consistent. The only difference for your size of the cube is to find something (a ruler, a strip of wood, etc.) that puts the dots as far from the center point as you like. Just try to picture it as a giant set of dice and go from there.
I then used a drill press to drill out 1/2" holes. I then used a hole saw on the drill press and used pine, oak (from pallet wood), and some scrap redwood from other projects around the house to make small plugs - almost as if you're sinking in screws and are making plugs hide hardware - that concept, other than if they don't cut clean, you don't have to worry. When you glue them in and when they're dry, you can simply sand 'em down nice and smooth, so don't bother doing much cleanup at the front end - it doesn't save you time. You'll be drilling out 21 plugs per cube - as well as drilling 21 holes in each cube to make a single complete die. You'll need five dices for a full Yahtzee set.
When all the dice were done, I used a belt sander (4") and ground off the plugs so that the surfaces were nice and smooth. I re-routered any corners that got impacted by overzealous sanding, and then hand-sanded to smooth them down so they'd be comfortable in small hands.
I stained some of the plugs for better contrast, but most I didn't bother - planning on using light plugs on the darker redwood cubes I had, and darker wood on the pine cubes. When the stains were dry, I applied a LIBERAL coating of Tung oil. Yes - I did NOT use polyurethane. I figured that these will be bounced around, will hit cement, rocks, etc., and if one of the recipients needs to re-sand a damaged corner, they could just apply a bit of wood oil, and they were back in business in minutes. I followed directions and used two coats, gently rubbing between each coat to absorb excess oil.
When the dice were drying, I built vertical crates for them (on purpose), sort of like a wine bottle bag out of pallet wood. These I did polyurethane, but on two of the sides, I created scoreboards using chalk paint (again - follow directions), and then drew the scoreboard boxes, etc., using a 7mm yellow acrylic paint pen because that stuff dries rapidly and would show up well.
I finished them off with some chunks of leftover sisal rope from my cat tree project to make little handles-slash-retainers. They're just bulky enough that if you knock the crate over, the dice don't go falling everywhere.
I made six sets, and my family/friends LOVED them! They were a huge hit, and a great way to use 30 cubes! :D The only thing I had to buy specially for this project was some chalk at the local dollar store (five multi-colored chalk sticks per pack, so 20 cents per Yahtzee set).
So hopefully, this gives you some inspiration for those leftover cubes. :-D