Aha! When I was planning my wedding reception a few years ago, I was also looking for a venue with pretty much the exact same specifications. I probably looked at at least 25 sites. I found that there aren't a lot of venues suitable for around 100 people, most sites work best for smaller or larger groups.

Alcohol is allowed at all the venues mentioned below. Typically you just need to get a permit and pay a larger deposit. Seattle Parks venues require that you keep the booze indoors, so no drinks on the patio.

The Alki Bathhouse (Seattle Parks) is lovely: right on the water, cozy indoor space (with bar area and kitchen), and a big outdoor patio adjacent to the beach. Current rates are $90 an hour, and with the extra fees (staff, booking, maintenance, etc.) it would be about $900 for a 6-hour event. Alki would be tight for 100 people, but it's perfect for around 80 (which, in my experience, is how many will show up if you invite 100.) I attended a wedding there recently and it was fantastic.

The Golden Gardens Bathhouse is perfect for 100 people, but an 8-hour rental (required Fridays, Saturdays, and Sundays) would run more than $1000. This is where the Mister and I ended up having our reception. (It was cheaper back then.) Although it sounds like this location wouldn't work for your friends, I thought it would be helpful to mention that working with Seattle Parks was great. The staff were helpful and unobtrusive, and there were no surprises on the bill.

The lodge at Camp Long in West Seattle has a great CCC vibe, but it accommodates a max of 72 people and doesn't have any dedicated outdoor space. (It would be perfect for a fall or winter wedding.) On the plus side, it would be well under $1000, and there are a number of cabins you could rent if your guests wanted to stay overnight.

Daybreak Star is a neat facility, but there isn't much dedicated outdoor space, and the interior is pretty cavernous; it seemed better suited to 200-plus people than 100 people. It would also be over $1000 to rent during peak times.

The Graham Visitors Center at the Arboretum has a gorgeous patio and was our second choice for a reception site. (It was second because the inside space isn't as neat as the interior of the bathhouse.) The rental rates aren't listed on its web site, but when I priced it a few years ago it was under $1000.

The garden patio at the Center For Urban Horticulture is stunning, but the indoor space isn't nearly as nice, so you'd be dependent on good weather. Like the Graham Visitors Center, rental rates aren't listed on the web site, but it was also under $1000 when I priced it for myself.

I do not recommend going the route of these people, but I'm sure your friends are far too civilized to consider such a thing.