First off welcome to the forum Ann or Anna. We are happy to see you hear and you asked some good questions and I am sure will get some good answers as well.
As far as services go, I only do hydroseeding so I don't have a lot of first hand experiences in the other facets of this business that some have found good. Hopefully some others will jump in with answers to that.
I do find the hydroseeding business to be good even though construction is down. I did probably about 80 jobs this year ranging from very small to some pretty good sized ones.
As far as promoting my business goes, over the years I have tried everything from pennysaver ads both classifed and display, yellow page ads, yard signs, mailings and just stopping and dropping off information at places under construction. When I first started I did a mailing to all the home builders and construction companies in my area and it was worth while. The pennysaver ads were a waste for me, and an expensive one but others have had good results. Right now I rely mosly on the IAHP listings and my web site which is one with free hosting through the IAHP and it pays off well for me. Most of my business comes from those. My continuing cost for the web site is about $ 10.00 a year so since it brings in 10's of thousands of dollars in business I have to think it is the best advertising money I spend. The yellow page ads used to work ok for me. They were expensive but they paid. I haven't done that in many years and I think they have gotten cheaper but are also not used nearly as much. Maybe the online Yellow Pages might be worth while but I haven't tried them to know.
As far as other profitable ventures go, it might help us to know where you are. For example if you are in Phonix we might forget about recommending snow removal.
I do think there is a strong trend to organics and some are doing well promoting organic hydroseeding and organic lawn care but I have not tried it myself yet to know. I will stop here and we will see what some of the others have to say.