1) Tiling. Learn to tile floors and either buy a wet saw or learn where you can rent one locally. Of course this is regional; I'm aware that people hate tile floor in cold regions because the floor is so cold when it's 20 below outside.
2) Drywall. Learn how to install and repair drywall, learn when other materials would be better (durarock, hardie, etc) and how to finish. I bought a simple Wagner texture sprayer that works great, but you have to thin joint compound to the point of it being as loose as a thick paint for it to spray well.
3) Basic electrical. Nothing crazy, but learn how to replace outlets, light switches, GFCIs, and how to install lights and ceiling fans. In my 3 house I replaced all of the outlets with new "safety" outlets (they prevent kids from sticking an object in an outlet and zapping themselves), and light switches with the big fancier Decora style switches. Buy the little tools to "sniff" if the wires are hot.
Other easy stuff:
* I run 2 complete coax cables into each bedroom and living room and 1 into the kitchen and other rooms. With the right tools working with coax is relatively easy. I rip out what is usually a spaghetti mess of wires and splitters in the attic with this method and then all wires run to a common point.
* Baseboards. There's no need to hire a carpenter. Just get a nice miter saw and use quality material.
* Landscaping. It's virtually impossible to screw this up yet investors frequently pay professionals. Visit a local Mom-n-Pop nursery and ask what will work in your area and do it. Sometimes the big box places stock plants not suitable for the store's region. Also do the minimal. If you're flipping the buyers may want to landscape themselves, and the juvenile micro-plants you planted were a waste of money.
* Replacing toilets. It's actually pretty easy. Also toilets are virtually indestructible so consider saving the toilet and replacing its guts. I cleaned hideous toilets back to virtually new in a few minutes using pool acid. I've replaced a lot of the guts in a toilet and the next time I renovate I'm going to fully rebuild a toilet with stainless steel: the only metal truly suitable for water exposure.
* Painting: unless you are so busy you literally don't have time to do this or you hate it with an absolute and irrational passion then you love throwing money away.