I got together with some good friends and former housemates a few years ago. We chose Seaside as our destination because it was on the coast and seemed like a very cute little town with lots of rental homes. It's a small town and therefore very walkable. We liked the cute main street with its restaurants, shops, ice cream parlors, and arcade. The Pig N' Pancake provided a hearty breakfast, but I highly recommend the Cajun restaurant slightly on the periphery. You can rent paddle boats for the river or scooters to zip around town in. We did both and LOVED the scooters, but you'd be limited to the boats if you travel there with kids.
All in all, we had a great time and liked the cottage we all rented together. The rooms were not equal, but we knew that going into it. We used and had a good experience with Beach House Vacation Rentals: http://www.beachhouse1.com/default.asp. Seaside is full of vacation homes and it seems that BHVR is a property management office for many of them. BHVR also serves Gearhart and the vicinity. Cannon Beach was an easy day trip from Seaside. We hiked there on our second trip to Seaside. Cannon Beach is supposedly a bigger town and seemed a little bit more adult a destination (something to keep in mind if you're traveling with kids).
Beach House Vacation Rentals has an office on the main road just as you're getting into Seaside. (The building is across from the plaza with the grocery store). We came into town late, but had been left a lockbox with keys and instructions (the code had been emailed to us, I think). There was the usual paperwork beforehand. They communicated with us by phone and answered all our questions promptly. The contract was very detailed and outlined most everything about the rental that we wanted to know (deposits, cleaning fees, etc). The only somewhat unusual thing was that they don't take credit cards, so we had to pay our security deposit, pet deposit, and the balance using checks. One person in our group was the point person (collected money, talked on the phone with the people at BHVR, etc).
The house we rented was cute if a little bit musty, but was spacious enough for the 8 of us. The house was very reasonably priced and a short half-block walk to the beach. It had an outdoor shower to rinse off beach sand. In addition, the house came with cooking implements and basic spices in the cupboards, a grill, and a ping-pong table in the garage (which, unfortunately, smelled like tons of animals had been locked in there while vacationers were out and about). For entertainment, there was a radio, TV, and a cupboard full of board games. We may have had to pay more to use it (I don't remember now), but there was even a nice washer and dryer that we could use.
The only thing that was a bit of a detraction from Seaside was that there were a number of dead birds and fish on the beach. I'm not sure what was going on there. It seemed worse the first time we were there and somewhat less noticeable the second time we visited a few months later. The beach is a bit cold for sunning, but we gave it a good go, built bonfires, and flew our kites.
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