So Jocchan tipped me off to this really cool game map-making program called
Tiled.
It will be really nifty for planning out levels on your computer.
Save this tileset I made somewhere on your computer:
It's every object in the game for the SMB1 ground theme minus the giant versions of enemies. I made some of the sprites that are unique to SMM like wiggler and Bowser Jr and borrowed some other from other games.
Open the program. Go to new or file > new to start a new map. Change the tile size setting for both width and height to 16px. The maximum size for a map in SMM is 257x28 tiles, so input those numbers in map size. You can make smaller maps if you want. The starting point of a map is 5 tiles wide and the end point is 12 tiles wide. Submaps can be as big as the main map minus the 17x28 tiles for the start and goal.
Once your map opens you can click on the background color option in the map properties list on the left to change it to whatever you want.
Now click on "map" at the top and click "new tileset". Click browse and open up the Mario Maker tileset from wherever you saved it.
You can now paint tiles onto the map just like in SMM. For sprites bigger than 1 tile you'll have to paint in each of their pieces, but since they snap to the grid it's no hassle. You can click view -> show grid to display the grid to give you a better idea of where it is. The program supports layers, so make multiple layers by just right clicking in the layers box and adding another. Use this to put tiles on top of other tiles.
The stamp tool lets you paint tiles tile by tile. The fill tool lets you fill an entire space with your selected tile. Use rectangular select to select a space to fill or to select a set of tiles you want to copy and paste.
That tileset should suffice for all planning needs. All of those objects have an equivalen in the other themes, so just use your imagination if you want to make a castle or underwater level with this.
When you are done with your map you can save it as an image like this:
(I cropped the rest of the map off)
Enjoy!