You'll find ATMs in Tallin. If your bank charges for the transaction, it might be a little expensive for just pocket money, but probably no more than at a currency exchange. There are so many Finns going to Tallin (at least there used to be) that I suspect that the euro will be accepted, with the merchant taking a hefty cut on the exchange; but it would be more convenient.

We had a day in Tallin, but 15 years ago.

Yes - Estonia uses the kroon - which appears as "EEK" on pricetags and amused us no end when we were there a couple of years ago ("That costs 50 EEK!" - we're easily amused). There are plenty of ATMs around so I'd advise taking out about USD$50 worth or so.

As for what to buy - outside the walls of the Old City is a nice market that sells knits and felted hats, gloves and so on - which make nice souvenirs and are lovely and warm for winter days. You can also buy attractive wooden toys for any children in your life. Many shops sell lovely table linens - well worth purchasing and they don't take up too much room in the suitcase.

One thing that struck us - Estonia is quite a poor country and we saw a lot of elderly women selling bunches of lily-of-the-valley to tourists. They just stand there all day with these little posies to sell. They're arranged quite prettily and they only cost about a kroon and the the ladies were heartbreakingly grateful for the money. Maybe consider buying a little posy to press in the pages of your book for a souvenir?

We were in Tallinn 18 months ago. Euros were accepted everywhere, including in the smallest restaurants and all shops. We were even able to use them at the post office from which we shipped artwork back to the US. There is no need to get local currency. I suspect that,for a day trip, it would not be economical to get Kroon at an ATM and then exchange the leftover currency when you return to Helsinki.