Aquatic plants are a great thing to add to your aquarium, they improve the water quality and make your fish happier. Here are some easy-to-care-for aquatic plants for beginners:
Anacharis - high light, can be floated or planted
Java fern - low light, can be floated, planted, or tied to rocks/driftwood
Anubias Nana - low or medium light, can be planted or tied to rocks/driftwood
Water wisteria - low, medium, or high light, can be floated or planted
Water sprite - low, medium, or high light, can be floated or planted
Java/Christmas/Spiky moss - low or medium light, can be floated or tied to rocks/driftwood
Marimos are a good choice!
I wouldn't recommend going to Petco or other big chain pet stores for plants, they often know nothing. Check to see if there's a fish store in the area, they often sell aquatic plants. If you have to order online, look for some of the less expensive options on Amazon, make sure to read reviews! If that's too pricey, a user on this site, JDAquatics, sells a lot of aquatic plants for really good prices, so private message him asking about it!
- The gravel should be fine for the plants, no need to switch to sand.
- I wouldn't get a snail, you don't want to overstock.
- The driftwood is leaching tannins (natural color from wood/leaves that lowers PH) into your water. Is it just something that you picked up, or did you buy it sterilized and ready to sink? The tannins will eventually go away if you soak it for a few weeks.