I have a boulder/rock that is about 2 feet long and 1.5 feet in diameter. It is too heavy to lift and I would like to break it into smaller pieces. Without power tools, are there any good methods besides going at it with a sledgehammer?
You can go at it with a sledgehammer (or a smaller hand sledge) and a star drill, and then drive wedges into the holes (or if you are patient, fill the holes with water and let them freeze in the winter.) There are special wedges designed for use in round holes for splitting rock (feathers and wedges seems to find them). https://en.wikipedia.org/wiki/Plug_and_feather
Remember to use ear and eye protection.
Another approach from our practical forebears if you just need it out of the way would be to dig a hole 2-1/2 feet long, 2 feet wide and 3 feet deep next to it, then roll it in and bury it.
You could also build a bonfire on or around it, but that has some potential to become more exciting than planned for if the stone has trapped water in it.
There exist expanding compounds like this one: Ecobust, which are poured into predrilled holes and expand as they dry, splitting the stone (or concrete). It does require a power tool, unless there are already some cracks in your boulder, but a cordless hammer drill should be sufficient.
Without (electrical) power tools, or a sledgehammer:
Here are 10 options besides a hammer or Fire-setting... mentioned in almost every post (but more specifically, for best results use at least 300 lbs of wood or 100 lbs of wood and 20 lbs of bbq briquettes; let it burn overnight and then (in the morning) dump 15-20 gallons of ice-cold water on it.)
Option 1 Use a thermal lance to punch holes in it or cut it apart in about 30 minutes.
Option 2 Use an oxyacetlylene torch to cut or drill holes into it in 20 - 60 minutes.
Option 3 A large fresnel lens with a good focal point on a sunny day will melt a hole in it in anywhere from 20 minutes to 20 hours.
Option 4 After using any of the above methods to create a hole, insert black powder or TNT into the hole and detonate it (or use ecobust as mentioned by mustaccio).
Option 5 Use a gas powered concrete saw to cut this rock (maybe 10-30 minutes).
Option 6 Use a bull dozer, back hoe, or other kind of excavator to remove the rock (20 seconds).
Option 7 Use about 80 lbs of Thermite to turn the rock into lava (keep the fire extinguisher handy).
Option 8 Use a catapult or rocket to launch the rock into a larger rock.
Option 9 Shoot it with a cannon.
Option 10 Call in an air strike.
Use a crowbar and put another rock under it so that sections of it are unsupported. Now whack it with the sledgehammer a few times.
If you get lucky, it will have a crack in it and you can break it into pieces. Otherwise you can knock off sections from the end.
Worst case, use the plug and feathers approach described above.
Another technique is to make a large fire (e.g. bbq briquettes) on top of the rock, let it get super-hot, and then douse it with water, causing it to crack.
Obtain a chisel with a rubber protector around the top, (more contact points and less chance of hitting yourself.) Proceed to hit this chisel with a rubber mallet or other leverage-providing, hard-ended object. By nature of the chisel's inclined plane, forward motion will cause the rock to crack and split along the business end's edges. Depending on the kind of rock, you will soon have a large enough area broken up to sufficiently hold the chisel without aid of your hand.
Now use your large implement, (potentially a sledge) to apply a significantly greater force than used before, (making sure to use protective equipment, and both hands,) and you will have a wonderfully large split, or at least a significant compromise of the rock's structural integrity. Repeat this process as many times as needed. Through this, with little expense, like using a wedge with an ax to split firewood, you can break the rock into small chunks, which can subsequently be taken away.
You will notice the appeal in using an incline plane to do the work for you, preventing injury and potentially larger amounts of expense.