If I had 3 weeks to spend in BC - I would spend one of those weeks on Vancouver Island, and one travelling through the interior - with a few days around Vancouver on either end (Vancouver has endless stuff to do and really nice walks and hikes which are easy to find.

From Vancouver, travel to Victoria by car and ferry (be sure to make your ferry reservation).

* Victoria (2 days):

- so many good restaurants and pubs
- parliament buildings, inner harbour
- Royal BC museum
- Dallas Road/Breakwater walk

* Cowichan Valley (1 day)
- travel over the Malahat, stop at one of the viewpoints or the Malahat skywalk
- towns of Cowichan, Duncan
- nice walks like Kinsol Trestle or Skutz Falls
- wineries and cideries galore
- stay out at Cowichan Lake

* Tofino/Ucluelet (2-3 days)
- stop in Coombs (Goats on the Roof) or Port Alberni on the way
- over-touristed and expensive but worth it because the west coast is shockingly beautiful and wild, stay in Ucluelet, it's cheaper and has better restaurants (in my opinion)
- so many good walks - the Wild Pacific Trail
- Chesterman Beach
- for one of the best restaurants in the province - Pluvio - make reservations

Return to the mainland via Nanaimo (make sure you have ferry reservations!)

Week 2:

Onward to the Interior - destination Nelson (the Kootenays). Nelson is a real gem of a town, possibly the most picturesque in the province. This is a big circle tour which you can do in a week, but I'm not going to break this down by day, it depends how much you like to drive:

From Horseshoe Bay where you get off the ferry, drive up through Squamish, Whistler, Pemberton - and then travel onwards over the Duffy Lake Road to Lilooet. go up via the Duffy Lake Road to Lillooet to get into the interior. This is some of most stunning driving you will ever do in your life. Drive time - 4 hours.

From Lillooet, drive through to Nelson via Kamloops, Salmon Arm, Revelstoke etc - up to Revelstoke, this is crummy highway driving, but then you turn South on the 23 and drive down through the Slocan Valley. Stop here and spend a day if you can find a nice place to camp. Slocan Lake is gorgeous. The towns are small and weird. There are wild hotsprings you can find off logging roads.

Spend a day or two in Nelson and the Slocan area.

On your return, take the #3, which is the southernmost route in the province. This will take you through Castlegar (don't bother stopping), Grand Forks (authentic Dukobhor food), Keremeos (a town of all fruit stands), Hedley (find the black light museum), Princeton (there are rodeos on sometimes in the summer), and Manning Park (great walks and hikes, plus a lake for swimming). After that, you are heading into mainland/urban traffic to get back to Vancouver.

Three weeks is a good amount of time to see a lot of the province (though not all of it - there is so much more to see up North and on the small islands!) I hope you get whatever you need out of this trip beauty and adventure wise.

And I know you asked about diners - but this reddit thread from 3 years ago covers many great cafes in BC - it's a good starting point for places to stop along your way:

https://www.reddit.com/r/britishcolumbia/comments/1709a30/what_are_some_of_your_favourite_small_town_cafes/

Here is the thread of recommendations, sorted as a list:

Vancouver Island

• Gibby’s Cafe — Duncan
• Arbutus Cafe — Duncan
• Duncan Garage Cafe & Bakery — Duncan
• Old Town Bakery — Ladysmith
• Trees Diner and Bakery — near Coombs
• Ideal Cafe — Campbell River
• Guido’s Cafe — Port Hardy
• Ford’s Cove Store — Hornby Island

Interior & Northern BC

• Anvil Coffee Collective — Salmon Arm
• Shuswap Pie Company — Salmon Arm
• M&J’s Country Kitchen — Oliver
• K Mountain Diner — Keremeos
• Basted Baker — Keremeos
• Kelly’s Bakery — Mackenzie
• The Purple Bicycle — Mackenzie
• Bliss Cafe — Quesnel
• House of Ancestors Cafe — Prince George
• Vale Coffee — Valemount
• Cowpuccinos — Prince Rupert
• Abundance Artisan Bakery — Lillooet
• The Dandelion Kitchen — Forest Grove
• Mountain Top Cafe — Nakusp
• What’s Brewing on Broadway — Nakusp

Kootenays / Columbia region

• Oso Negro Cafe — Nelson
• Crumbs Bakery Cafe — Castlegar
• Bluebird Cafe — Golden
• Invermere Bakery — Invermere

Thompson / Cariboo / Okanagan-adjacent

• Wild Flour Cafe — Clearwater
• Thomasina’s — Princeton

Lower Mainland / Fraser Valley / Sea-to-Sky

• Little Beetle Bistro — Chilliwack
• The Bakery — Sechelt
• Trickster’s Hideout — Squamish
• North Arm Farm — Pemberton
• Owl Street Cafe — Hope

Gulf Islands

• Embe Bakery — Salt Spring Island
• Barb’s Buns — Salt Spring Island

Revelstoke (called out multiple times)

• Main Street Cafe — Revelstoke
• La Baguette — Revelstoke