booking cuba tour guides (not a full tour)

Try the Lonely Planet Travel Guide and web site (yes there are other guides out there not 99% oriented around travel to Italy France and Germany and you will get a lot of responses).

Also, since you are a USA citizen visit the state department Cuba site to see the regulations.
You will see as an average US citizen there is no way you can go legally inexpensively.

Recent PBS Newshour discussion seemed to indicate that Trump is undoing President Obama's loosening of Cuban travel restrictions:
http://www.pbs.org/newshour/bb/changing-tides-u-s-policy-may-sink-cuban-tourism-hopes/

Mr. Trump’s restrictions are expected to be spelled out next month.
But many Cubans are already concerned that their hopes for better
relations with the U.S., and greater economic opportunity, will now be
set back.

You should check with the American State Department about Cuban travel:
https://cu.usembassy.gov/u-s-citizen-services/local-resources-of-u-s-citizens/traveling-to-cuba/

Travel to Cuba for tourist activities remains prohibited by statute.
However, the Department of Treasury’s Office of Foreign Assets Control
(OFAC) has issued general licenses for 12 categories of travel.

You can also go on a vacation to Mexico and do an Internet search on "us citizens entering cuba from mexico" or variations of the topic. You may also want to bring Euros or Canadian Dollars for your side trip.

If you are still looking for things to see/do in Cuba I highly advise you to look into scuba diving! Cuba has some of the best spots in the world for that and it's not very expensive, depends on where you want to do it. if you want to take it into consideration here's a helpful link for the best spots to scuba dive there, I hope it will be of help
https://tripblan.com/trip/best-scuba-diving-places-in-cuba
Cheerio!