Must haves?

Also, this might be a little off topic, but what is a good yet cheap scale to use for weekly weighing your guinea pig?

Mine love little paper bags stuffed with hay or veggies. They also like toilet paper tubes stuffed with hay. You can get hanging ball baskets to load with goodies and hang in the cage as well. Wadded up paper to push around is another diversion. Cardboard boxes with entrances, exits and escape hatches are a "must". The things that they seem to enjoy the most are cheap or free. Just make sure that it is safe for them to chew on, because they will. As for a scale, an inexpensive kitchen scale from WalMart, Target, etc. works fine. One with a bowl is probably a better choice so that your piggie is "contained".

Chew toys, a tunnel, a cozzie, and a hidey house. Also when you give them veggies try new ways every day. Also have some toys.

A hidey (or 2 or 3 depending on hom many pigs you have) and a cozy (or more than one depending on how many pigs you have)

Chew toys aren't a good idea as most of them are dyed and they're completely unnecessary (as long as the pigs have unlimited grass hay they're teeth should wear down fine).

My pigs like anything they can run through: tunnels made out of bent grids covered in fleece, cardboard boxes with lots of holes, scraps of fleece strung across the cage, corner tents made of fleece, ect. I also have a wind chime made of metal that they cannot chew apart that's hung just high enough in their cage that they can ring it. The noise excites some pigs and scares others though.