I'd suggest kattis and CodeForces. I use kattis mostly, but know a number of people who like codeforces better.

Here's a few for you to try. Some of these are pretty hard, but you should be able to find solution sketches online if you google the contests they are from.

https://open.kattis.com/problems/increasingsubsequence

https://open.kattis.com/problems/maximumsubarrays

https://open.kattis.com/problems/tray

https://open.kattis.com/problems/dinnerbet

https://open.kattis.com/problems/hyperpyramids

I've solved all of these as well, so if you get really stuck feel free to reply here and I'll try to guide you through them.

Thank you. I will definitely be coming back. I guess I have difficulty formulating a DP problem properly.

I found another recently that has DP on a tree:

https://open.kattis.com/problems/theescape