Equipment (this can be divided into 3 experiments - you can do some or all of them) :

Salt, Wire wool / fine sandpaper

Experiment A (easy to do at home):
2 equal pieces of aluminium kitchen foil
2 glasses

Experiment B (a bit more fiddly):
2 Iron nails
Flexible plastic lid (one from a milk carton is ideal)
Small artists paint brush
Hairdryer or electric heating fan

Experiment C (even more likely to make a mess):
2 Iron nails
Some modelling clay, plasticine, or blu-tak
White oil based gloss paint and small brush
2 glasses

Method:

Experiment A: Take two glasses and half fill them with water. Mix two spoonfulls of salt into to one glass, and leave the other with fresh water. Put one piece of aluminium foil into each of the glasses and allow it to sink (make sure that you do not contaminate the non-salty aluminium foil or water with salty fingers!). Observe the glasses after a few hours; take notes of what you can see. Observe after 18-24 hours; take notes of what you see. You can take the aluminium foil out of the water to take a closer look.
If you see corrosion, what is the colour is the corrosion that you see? Rate the level of corrosion of from 0 to 5 (0 being no corrosion, 5 being very corroded).

Experiment B: Clean each of the nails with wire wool or fine sandpaper. Push the nails through the plastic bottle cap from the bottom so that they stick out like an old fashioned TV antenna (This is just to hold the nails in the air - if you cannot find a soft plastic bottle top, use cardboard or a lump of modelling clay instead). Clean and wash the paintbrush thoroughly. Wet the bristles and brush one of the nails with plain water. Then dip the wet brush into salt. Carefully "paint" the other nail with the wet salt until you have a good amount sticking to the nail (be careful not to get any salt on the first nail). As soon as you finish painting, dry the nails quickly using the hairdryer or heater until both nails are dry. The salt on the second nail should form a white crust.
Leave these nails standing in an inside room for 1 to 5 days. Observe them after a day; take notes of what you can see. Observe after 4 or 5 days; take notes of what you see. If you see corrosion, what is the colour of the corrosion that you see and where is it strongest? Rate the level of corrosion of from 0 to 5 (0 being no corrosion, 5 being very corroded).

Experiment C: Clean each of the nails with wire wool or fine sandpaper. Take two glasses and half fill with water. Mix two spoonfulls of salt to one glass, and leave the other with fresh water. Add a nail to each. Leave to corrode for one week. You have to remember which nail is which, so label the glass containing the salt.
After one week, take out each nail (be careful not to contaminate the non-salt nail with salt) and dry them on seperate pieces of tissue paper. Remove loose rust using the paper - then allow the nails to dry.

You must keep track of which is the salty nail, and which is the fresh nail - you need to know this all the way through the experiment, don't mix them up !

Once the nails have fully dried, push them into the clay (This is just to hold the nails while we paint them and allow them to dry - you can use any method you like to do this).
Paint the top half of the nails smoothly and evenly with the gloss paint, making sure that you have a full coat. Allow the paint to dry fully (read the instructions on the tin). Once the coat is dry, ensure that the paint covers all the metal (touch up gaps if required). Take the nails out of the clay when dry, and push the apined end into the clay. This allows you to paint the other half of the nail and produce 2 completely paint sealed nails, but one has been corroded in salt, and one has been corroded in fresh water.

Rinse out the glasses thoroughly, and half fill them with tap water. Add a painted nail to each glass, and record which is the salt nail, and which is the fresh nail (put a label on each glass to show this)
Leave these nails standing in an inside room for 6 to 8 weeks (or more - keep topped up with water if required!). Observe them occasionally during this period; take notes of what you can see. If you see corrosion, what is the colour of the corrosion that you see? Rate the level of corrosion of from 0 to 5 (0 being no corrosion, 5 being very corroded). If you see other changes, not them down.

After the experiment is over, you may want to take the nails out of the water and scrape away some of the paint to see what is underneath. Note if it is easier to remove paint from either nail, and what is the condition of the metal below the paint.