Estonian tourist activities you can forever brag about to your mates

Finally my mate and I booked a five day trip to Estonia in May. We've been talking about this for quite a while and we're so happy to go. We are, let's say, from one of the most well organized (some might say boring) countries in Europe (Netherlands). We're staying in Tallinn and we'd love to go for one or two trips in Estonia that are SO cool that we will remember this for years. What direction we're looking for? We went Urbexing in a deserted subterranean bunker complex in Moldova, went to Chernobyl, went shooting with PPsh's in Belarus. Curious as to what the great people of Estonia would recommend!!