Agility can be very expensive, but frisbee is a relatively cheap choice in dog sports.

Fostering is good if you like solving problems, want experience with a variety of doggy issues, have the means to do it, and wish to do a good deed. I personally think it's a wonderful experience, but realize that many of these dogs are severely lacking in manners. That can include things like going to the bathroom in the house and destructive chewing, so it can cost money.

The plus side is that you learn a lot from it and are far better prepared to own your own dog. You get to 'shop around' and own various different dogs, which can ensure that you eventually pick a dog that you connect well with. You can also experiment with doggy sports with fosters.