Tax-free shopping: foreign tourists are eligible to reclaim tax (VAT) on purchases of domestically-manufactured goods with a value over ARS $70 when made at participating outlets. The Global Blue website has useful information on the tax reclaim process here: http://www.globalblue.com/customer-services/tax-free-shopping-in-argentina/. AFIP, the Argentine tax authority, also has information in Spanish here: www.afip.gob.ar/turismo/.
Paying in US dollars: some restaurants, hotels and shops may accept cash payments in US dollars with a favourable exhange rate. It is always worth asking.
Keep some small change handy: many shops often run low on small change - both coins and small denomination bank notes. They will be very happy with you if you can pay with the exact change.
If you have a large amount of peso left at the end of your visit, you can chance them to US dollars at an official casa de cambio or in a bank (it may also be possible to exchange them for euros, or Brazilian reales, but other currencies are not regularly stocked). You may be asked to provide receipt from your original sale of dollars in orderto buy back dollars. If you have less than ARS $1,000 left, it’s probably best to use it for transport to the airport or for buying souvenirs.