As a soloist? There's a trombone and band feature called Paceline that would work with baritone.
I agree with the person below that some of the King marches have some great Euphonium lines if you're trying to just feature the section.
For tuba, on the easy side (JH side) there's a version of Wizard of Oz in 3 or 4 movements (can't remember arranger) that has a big tuba feature for if I only had a Brain. There's also the James Barnes version that features tuba there too at the Grade 4 level.
Randall Standridge is making a concerted effort to give tubas some better parts too - you might ask him if he has some suggestions.
