Yes, a UNION is the way to go:
SELECT 'comment' AS Type, Comment AS Content, NULL as Item_ID, Date
FROM Comments
WHERE User_Id = 1
UNION ALL SELECT 'like', NULL, id_of_liked_item, Date
FROM Likes
WHERE User_Id = 1
That will give you a result set that looks like this:
Type Content Item_ID Date
------- ------------------ ------- -------------
comment Hi.... null 24 March 2013
comment Another comment... null 26 March 2013
like null 101 24 March 2013
like null 145 25 March 2013
And you can take it from there.