There are many options depending on your skills and interests. The important objective is to find a service or product that someone (target market) wants (a pain point or desire) and is prepared to pay for.
Starting out it's best to look for a narrow segment of a market that is not being well serviced.
Business
Technical writing
Copywriting and editing
Project management
Strategic / management consulting
Business writing (business plans, grants, white papers, proposals)
Public relations
Research
Transcription / scoping services
Human resources / payroll services
Event planning / promotion
Affiliate marketing
Data entry
Bookkeeper
Create an online fashion boutique
Start a drop shipping business.
Create a membership program
Join an affiliate marketing program
Start an online coffee shop
Sell your photography
Phone accessories
Invest in vending machines
Investigate vendor financed business opportunities
Start a beauty business (nails, hair, makeup)
Creative
Video production / editing
Design (brochures, business cards, logos, newsletters)
Web design
User interface & user experience (UI/UX)
Audio production
Photography
Presentation design (PowerPoint, etc.)
Marketing and Sales
Sales
Internet marketing consulting
Search engine optimization (SEO)
Pay-per-click advertising (PPC)
Google Analytics
E-commerce consulting
Lead generation
Community management / promotion
Blogging / blog consulting
Small business marketing
Marketing local businesses
Become a social media management consultant
Social media specialist
Information Technology
IT Security
Developing apps
Testing games
Build websites
Computer networking
Database administration
Programming
Building WordPress web sites
General computer help / training
Create digital products or online courses
Test app development
Tutoring and coaching
Education (College/SAT prep, Math and English, Language, LSAT prep)
Productivity coaching
Sports / Personal Training
Business training
Martial arts instruction
Dance / choreography
Dating / social skills
Music
Financial
Accounting / bookkeeping
Tax and financial planning
Manufacturing and Repairs
3-D Manufacturing
Craft making (soap/candles/knitting/jewelry)
Food products (preserves/precooked meals/sauces/biscuits/cakes)
Cake decoration
Pottery
Weaving
Recycling waste products
Carving
Furniture repairs
Bicycle repairs
Automobile / Motorcycle repair
Design and sell print-on-demand T-shirts.
Sell print-on-demand posters, greeting cards, and prints
Sell handcrafted and homemade goods
Home Services
Cooking / personal chef
Precooked meals
Pet services (grooming, walking, sitting, etc.)
Home maintenance / organizing
Childcare
Gardening services
Swimming pool maintenance
Start a landscaping business
Become a handyman
Housesitting
Other Services
Travel Planning
Personal / virtual assistant
Courier services
Career help (finding work, optimizing resumes, cover letters, interviews)
Fashion / image consulting
Physical therapy
Public speaking outreach
Launch your own book
Start a charitable business
Virtual event planner