1. Applying a layer of store bought bark mulch on walking areas,
If you want to keep the weeds down in the pathways, apply a thick layer. I would choose store bought mulch that was not dyed. If you have a lot of mulch needed, you can buy it in bulk from some speciality suppliers. Make sure that it is organic and has not been sprayed upon.
2. Applying a layer of leaves on growing areas,
If you have access to lots of leaves, this is a favourite choice of mine because I can find it in large quantities already bagged up by my neighbours in the fall. Is is so much easier to find bagged up leaves in the fall than to fine a large source of wood chips. I also get wood chips from the local wood lot what sells firewood.
3. Throwing some coffee grinds all over the garden, and
It is hard to have enough quantities to compare with bags of leaves, but if you have them, then use them in the garden or compost them. A variety of bio mass is good.
4. Interplanting some nitrogen fixers with my vegetable crops?
It can be hard to work with nitrogen fixers trees in a vegetable garden as the roots are usually in the way and likely the amount of nitrogen from the roots of these trees is not enough for the vegetables.
You could grow some herbacious nitrogen fixers in the vegetable patch, but it does take a lot of space that you might prefer for vegetable growing. If you have long growing season, you could grow this green manure (clover etc) in the off vegetable growing season and then cut it back into the soil just before planting your vegetables.
You could add compost at the beginning of the season. If you have access to quality manure, then add that in the fall.
There are many ways to get a good growing garden. The key is not in one "perfect" method, but to do the method that works in your context and what resources/imputs you have easily available.