Specialty tours are small almost by definition although nature & educational tours might be larger. Many companies offer or can arrange …. specialty group activities or focus tours.

A. Relationship tours: 
     1. Family, Siblings, Friends-Only:.re-new old flames, heal old hurts & share
         new stories.
     2. Adults only: avoids family/kid mayhem TO enjoy “adults-only” activities.
     3. All-women / All-men: Girlie things vs guy stuff.
         NOTE: NOT intended for European style “I’m free!” raucous bachelorettes or bachelor bacchanals which many hostels outlaw.
     4. Wedding: I am a bachelor, so, haven’t a clue.?� 

B. Sports & Adventure Tours: Any athletic activity you can imagine or design!    

Medium fitness to high intensity athletic ‘extreme sports’: road cycling & rugged mt bike trails, highly skilled white water river kayaking, camping & rugged, long-distance trekking, &  mountaineering. 

C. Activity Specialty: Hobbies & Interests: art & architecture, cooking, music, history, science, birding. Artisans of Leisure:  https://www.artisansofleisure.com/tour/Arts_tours_luxury_travel.html),

D. Earth’s Nature & sustainability/environmentalism: Such tours include Wildlife Conservation Adventures, Eco-Friendly Wilderness Exploration, Cultural and Community Engagement, Climate Action and Carbon-Neutral Travel, Regenerative Nature Retreats, & Educational Eco-Tourism 
     National Geographic tours &  Rick Steves’: 
          https://www.ricksteves.com/travel-tips/trip-planning/eco-friendly-travel-europe

E. Volunteering: Some tour operators promote volunteer experiences & pro-active sustainability travel (eco lodgings, etc) all over the world. 

Some Volunteer opportunities are for a few days or weeks to months. Curiously, some often charge a weekly fee; $180 > 3000/week or you work for basics: food, tools, experiences. Huh??? Go figure.   
            International Volunteer HQ (IVHQ): https://www.volunteerhq.org/destinations/

F. Specialty or Narrow Focus: Japan’s “Cherry Blossoms” tours: ,

Religious, Shopping, Gstronomic (food), Genealogy, Christmas Markets Getting-Back-To-Your-Roots.
    Trafalgra Tours: Japan’s “Cherry Blossoms” tours, 
    Vivator: Top Europe Ghost Tours:https://www.viator.com/Europe-tours/Ghost-&-Vampire-Tours/d6-g4-c118 

G. Curious, esoteric, contrived, silly Trips:
    1. Single, Yet Ready-To-Mingle Trip:      Huh?
    2. Do-Nothing-At-All Trip: go to Motel 6 & watch TV; save money & hassle

Travel Triangle: “15 Types Of Trips For Those Who Believe
Change Is  Only Constant In Life!”
https://traveltriangle.com/blog/types-of-trips/  
                                    [plpk: sub-pil” xyz Kinds of (B): Tailored Specialty Tours]
                                    [pplk: SIT Expanderr Tour Factors]

H. Peer-to-Peer ...local private host & guide.  A relatively new peer-to-peer local private host & guide option is growing as travel demand has shifted from ‘destination’ to ‘experience’ based vacations.

     Reuters Events:  destination to experience-led vacations,  

Tens of thousands of hosts in 30 cities across Asia and Europe & expanding to more cities; local hosts and local sites & experiences; often language tailored i.e. English for Brits, Americans, Aussies & Kiwis, etc. 

Private host’s unique, personalized itineraries, sites & activities tours offered directly to passion-driven travelers at an affordable cost, but focused on off-the-beaten-path experiences: actual local events, family dinners, etc. sites & activities. 
     a. Withlocals.com and heal2go.com
     b. Reuter Events - Travel: https://www.reutersevents.com/travel/social-media-and-marketing/bye-bye-mass-tourism-hello-healing-touch
     c. Crunchbase: 1,200 fully customizable, 100% private tours & activities led by 900 local hosts in dozens of international cities.
     NOTE: Lodging may or may not be included, but Airbnb Experiences, Vrbo etc. lodging sites can easily sync to coincide with peer-to-peer tours.