WHEN is "later on"?
The local joke here is "Summer starts on July 5th".
You can generally be assured the weather will be really crappy - or absolutely fabulous - during May and June. It's always a **** shoot.
There are a couple big "events": SeaFair is the first week of August (not sure on dates I always make it a point to stay far away.)
Bumbershoot is Labor Day weekend. (Was fun when I was in my 20s. Not fun for an old man - crowded, noisy, and REALLY spendy now.)
We don't have anything here along the lines of Harry Epstein.
Avoid the "duck boats" down on the waterfront - they kill people.
The museum at Volunteer Park has the largest and most extensive collection of Chinese jade artifacts on the planet outside of mainland China. (The Chinese Ambassador made his home in Seattle, Washington and donated his entire collection upon his death.)
There are also a boatload of other Chinese objects as well.
The University of Washington arboretum is interesting if you're into plants and trees.
South Seattle Community College Arboretum is pretty fabulous:

Yes, that is indeed a four-ton bronze carp.
Lincoln Park is overrun with dogs. Discovery Park is usually pretty crowded on nice days, but not so many dogs.
Seahurst Park (Burien, Washington - immediately south of the City Limits of Seattle) is the second-largest municipal park on Puget Sound and offers a couple options for "loop trails" through some fairly fabulous second-growth and a nice piece of northwest-facing beach.
There is some BIG money locally which supports Seattle Opera, Pacific Northwest Ballet, and the Seattle Symphony. There is also a Seattle Youth Symphony. The acoustics at McCaw Hall are as good as you will find anywhere on the planet.
It's been years since I've set foot in the Seattle Art Museum, but it might be worth looking at.
The "International District" is one of the largest "Chinatowns" in the continental U.S. outside of San Francisco. I always liked "Tai Tung" (655 So. King) because they stayed open really late - after the opera or ballet - and they have 19 million items on the menu. DO NOT order off the menu! Walk up to the counter and read all the paper signs on the wall and order THAT stuff instead. The mens room is as bad as it gets. Piss in the alley - it's cleaner.
"Kau Kau Barbecue" (656 So. King) is right across the street from Tai Tung. Fabulous. Not open as late, but they used to have "Chi Chi Chicken Hot Pot" and "Sweet and Sour Ginger Duck" - both of which were fabulous and neither of which are on the menu at Tai Tung.
The bathrooms don't smell as bad as at Tai Tung.
AVOID the Space Needle unless you can finagle a free ride up and down the elevator. The restaurant at the top is obscenely overpriced and the food is lousy, per the last oral report I heard.
There are hundreds of really fabulous places to eat in metro Seattle. AVOID the chains - there is NO reason to eat crappy food here.
30 miles south (and almost an HOUR drive, depending on traffic) is beautiful downtown Tacoma. TACOMA is host to the largest car collection on the planet: the LeMay Car Museum ( 2702 East D Street). MOST of the collection is actually out in Spanaway on the old Marymount Military Academy campus - they rotate cars in and out of the collection.
ALSO in Tacoma: Tacoma Glass Museum. Tacoma Art Museum (both well funded and always fabulous.)
And Pt. Defiance Park - second-largest municipal park in the continental U.S. next to Central Park in NYC. (762 acres, the outer end of which is old, uncompromised, low-elevation temperate rain forest. (Pretty fabulous and well-funded zoo and aquarium. We have polar bears and musk oxen!)(Okay, okay... Woodland Park Zoo in the north end of Seattle has a "butterfly house", but nothing says "cool" like Musk Ox.)
Snoqualmie Pass is a 45-minute drive (at 70 mph) up I-90.
Mt Rainier National Park is about a two-and-a-half hour drive from metro Seattle, and you have to make a reservation to get in unless you get there before 7:00 am.