Help with sound treatment for general music room

[equinox.fish]

In my home I have a general purpose music room (It is NOT used for mixing). My knowledge of acoustic treatment of rooms is scarce. The activities which i believe would be most affected by the sound of the room are rehearsals and drum recording. What are some cheap and easy ways I could improve the sound of my room? By cheap I mean under 200$

Room description:

The room is about 15x11 ft. One of the longer sides has one irregularity on each of it's farthest ends. The first is a short hallway 4 feet long that leads to a door. The second is a 11 ft hallway containing a small nook. All walls are standard drywall. 2 walls have large windows near their intersection point. The room contains a slim tall shelf, a futon sofa which covers most of a wall and extends into a corner. It also contains a home digital piano, a desk, and a 2 tier keyboard setup.

I just want my room to have a tighter sound and be more pleasurable to play and rehearse in. And I want less reverb in my drum tracks. I notice that when I clap in that or any other un treated room I can hear a sort of high frequency fluttery tonal reverb. Remember, I am not using the room for mixing or intense listening, so I don't need instructions on setting up monitors or placing absorbing panels o specific reflection points. I just want it a little bit mor professional. Nothing too fancy.

Hopefully someone can help me. Thank you!