Technically, one who wishes to drink after the first of the four cups may do so, but one should preferably not drink between the first and second cups so that he does not become intoxicated to the point of being unable to read the Hagada with proper concentration. However, it is permissible to drink non-alcoholic beverages such as grape juice or any other juice (SA 473:3, MB 16 ad loc.).
If, while reciting the blessing over the wine, one had in mind to drink other beverages, he need not recite a berakha over them, because the berakha over wine covers all beverages. Even if he did not intend to drink other beverages, but they were on the table and there was a chance that he might want to drink them, he need not recite a berakha before drinking them, because the blessing over wine covers these too (see SHT 473:18).
A firstborn who fasted on Erev Pesaĥ, or someone so hungry that he finds it difficult to concentrate on reciting the Hagada, may eat foods such as eggs, fruits, potatoes, and kitniyot (for those who eat kitniyot on Pesaĥ) after kiddush. However, he should not eat too much, so that he saves his appetite for the matza. Furthermore, this is only permitted when there is great need. If one can restrain himself, it is best not to eat anything before the meal, because when one eats after kiddush, he runs into the problem of whether he has to recite a berakha aĥarona (and if one eats, he must recite a berakha aĥarona).
This eating and drinking is only permissible before one pours the second cup and begins reciting the Hagada. However, after beginning the Hagada, it is forbidden to interrupt by eating or drinking, for reciting the Hagada is like prayer, which cannot be interrupted (BHL 473:3, based on Ramban and Ran; however, it notes that Ha-ma’or and Tosafot permit).
. According to SAH 473:13 and MB 479:5, if one wants to drink “ĥamar medina” (the alcoholic non-wine beverage locally considered significant), if it requires a new berakha, the beverage is prohibited, since be-di’avad one can fulfill the mitzva by drinking ĥamar medina for all four cups. This is the proper custom. (Ĥemed Moshe forbids even other drinks if one must make a berakha on them. See Kaf Ha-ĥayim 473:40. It remains unclear whether this stringency applies to the Sephardic custom, according to which a berakha is not recited before every cup.)
. One would presumably need to recite a berakha aĥarona. Since the food one eats before a meal is not considered part of the meal, he must recite a berakha after finishing the food before beginning the meal. This is the opinion of Ben Ish Ĥai (Naso 4), Kaf Ha-ĥayim 177:7, and Or Le-Tziyon 12:7. In principle, MB 176:2 also rules this way, but states that according to several poskim, if the food is of a type that is not covered by the berakha of ha-motzi, such as fruit, and the person intends to continue eating fruit during the meal, he should not make a berakha aĥarona, since the berakha that he made on the fruit will include the fruit he eats during the meal. Also, since Birkat Ha-mazon covers the fruit that he ate during the meal, it will also cover the fruit he ate before the meal. Consequently, an uncertain situation arises, since according to Rashbam the berakha on the karpas is also supposed to cover the maror. Therefore, SA states that one should specifically eat less than a kezayit of karpas so that he will not have to recite a berakha aĥarona. If one recited a berakha aĥarona over the food he ate after kiddush, that berakha would cover the karpas, and he would be required to make a new berakha on the maror. Perhaps he can have in mind to cover everything except the karpas with his berakha aĥarona, but this demands further investigation. Perhaps we may say that since he is eating after kiddush, the food is considered part of the meal and is covered by Birkat Ha-mazon. The only problem is that according to many poskim, including Ben Ish Ĥai and Kaf Ha-ĥayim cited above, Birkat Ha-mazon does not cover anything eaten before the meal. Moreover, even those who generally maintain that Birkat Ha-mazon covers food eaten before the meal, at the Seder there is a large break between the food eaten after kiddush and the meal, so it would seem that everyone would agree that in this case Birkat Ha-mazon does not cover anything eaten before the meal (Yeĥaveh Da’at 1:2). Indeed, Birkat Ha-mazon covers the wine that one drinks for the first cup, but this wine is different, because it is certainly connected to the meal, since the recitation of kiddush over this wine specifically allows one to partake in the meal. On the other hand, food that is eaten after kiddush might not be connected to the meal. In practice, one who eats after kiddush must recite a berakha aĥarona (“borei nefashot”), but should not recite a borei pri ha-adama on the maror. See R. Harari’s Mikra’ei Kodesh ch. 4, nn. 141 and 142.