Varied diets and different sized perches are also very important.
Perches of different diamters keep the birds from getting arthritis because it stretches their feet out.
Just a seed diet isn't good. Seed mixed with dried fruits and vegetables is good. Also a treat cup with fresh veggies. Zupreem is also a very good food to mix with seed.
Avoid toys that have a lot of rope because they can get tangled and break their legs or toys with small holes to get caught up in. Also be careful what type of bedding you buy. Corn cob is okay as long as their is a grate so the birds can't get to it. If they eat it at can cut their insides and kill them and or block their intestines because they can't pass it. Pine and Ceder bedding is very bad because it has a strong scent that damages birds lungs and gives them resperatory problems. Plain computer paper, not newspaper is good. Newspaper has ink that can damage the bird if they deside to rip it up. Aspen bedding is what I use because it has no harmful oils or odors like other wood beddings.
For one parakeet I would say a cage at least two feet long. Gives room for pleanty of pearches, toys, and room to stretch. Also leaves space for another keet if you ever decide to get another.