Hello, City lass.
I went on this holiday at the beginning of September, been home a fortnight ago now. I haven't written a review because Roger the Red did one after his trip last year (go into "Search", "Reviews", "Baltic States" to find it).
I thoroughly enjoyed the holiday although by the end I could only work out what I had seen when by looking at the itinerary! Our TM who had a steps app on her phone calculated that we had all done an average of 14,000 steps a day! The main roads were almost all good, resurfaced with EU money, but the minor roads and paths in the towns and villages are generally composed of cobblestones, some made with local clay but others just stones which the locals had to go out into the fields to find in Soviet times, so they are very uneven under foot and it is essential to have stout shoes or walking sandals, no summer sandals or flipflops. Besides all the walking there is a lot of sitting in the coach through flat countryside but we never went for more than an hour or so before pulling in for a comfort stop and, usually, time to get a snack at a petrol station.
When it comes to packing, I took just about everything including a woolly hat and gloves because the weather forecast over the 3 countries went between sun and 24 degrees in Vilnius to rain and 13 degrees in Tallinn. In the event, we had generally good weather (t shirts and sweaters) although it got cooler by the time we were in Estonia and we only had one day of heavy rain, in Tallinn, plus one tremendous thunderstorm overnight in Vilnius. As you will be there in mid Autumn I would suggest you take layers to cover every eventuality.
One final thought - you might want to take insect repellant with you. I don't know when the mozzies and sandflies die/go into hibernation but several of us got bitten by them in the National Park and needed to apply antihistamine wipes or sprays right up to the end of the holiday. Better to take them and not need them than to itch and scratch.
Have a lovely holiday.
Ann
PS You have a long wait in Frankfurt airport on the way home. Bottles of water cost at least 3 euros (including a deposit of 25c) but there is a water fountain in the Departures area so if you take an empty water bottle in your hand luggage you can fill up for free. We didn't have so had to pay and of course won't get the deposit back.