Buying vouchers for other weeks are definitely something that is high priority, especially to get the ''Obtain (profession) vouchers" Achievements, as buying them counts as obtaining! (you can also buy them to get more tickets for a certain companion from another weekly activity!)
After that's over, something i do personally is buy bones (considering you get 10 bones per ticket which is a great deal in my book) or get a bunch of cooking junk :3